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The MiniSwitcher gives a regulated output from 1.2-20V at currents up to 1.5A and doesn’t require a heatsink. By NICHOLAS VINEN Simple 1.5A Switching Regulator This tiny regulator board outputs 1.2-20V from a higher voltage DC supply at currents up to 1.5A. It’s small, efficient and cheap to build, with many handy features such as a very low drop-out voltage, little heat generation and electronic shut-down. I N THE DECEMBER 2011 issue, we presented the MiniReg, an update to our LM317-based 1.3-22V adjustable linear regulator. This has been a very popular kit over the years because it’s cheap, simple and can be adjusted to suit whatever voltage you need. But while an LM317 regulator circuit might appeal to old dudes and codgers, it’s so “1980s”! For anyone in their thirties or younger, it’s just plain boring. In fact, the LM317 was designed in 1970 by two engineers working for National Semiconductor. That’s over 40 years ago, well before I was born! And while linear regulators are still in use in many applications (yeah, yeah, still boring), these days, Main Features • • • • • • • • Wide operating voltage range Very low drop-out voltage High efficiency No heatsinking necessary Electronic shutdown Thermal, overload and short circuit protection Soft start Provision for power switch & LED 64  Silicon Chip just about every computer, monitor and TV (and a lot of other gear) uses switchmode regulation. The benefit of switchmode regulators is much higher efficiency. This means lower power consumption, less heat and cheaper components (eg, smaller transformers and heatsinks etc). Small size, light weight and low power consumption are particularly important for portable electronic gear. In short, for a large range of applications, why would you bother with linear regulation? Linear regulators only have to be used if you need very low noise and ripple and for EMIsensitive applications like radios. For just about everything else, switchmode is the way to go. Just look at the photo towards the end of this article – it shows how large a heatsink you need to get the rated current of 1.5A from the MiniReg with a 14.4V input and a 5V output (ie, when the power source is a lead-acid battery). That is no longer a small or cheap regulator! Then there’s the fact that a lot of linear regulators have quite a large “dropout voltage”. This is the minimum difference between input and output voltage. For example, to get a regulated 12V, you generally need at least a 15V input (unless you are using a low-dropout regulator or LDO). If you are using the MiniReg as a speed controller for the Magnetic Stirrer project in the December 2011 issue, you can’t run the fan at full speed if you are using a 12V power supply. In that application, it isn’t a problem but sometimes the high drop-out voltage is a serious inconvenience (and it increases the dissipation as well, because the supply voltage is higher than it would otherwise need to be). Enter the MiniSwitcher With only a modest increase in size and complexity, this design overcomes all those limitations. Like the elderly LM317, the chip we use here (the AP5002) has an adjustable output voltage, can deliver around 1.5A and it also has over-temperature and over-current protection. But unlike the LM317, it has a very low dropout voltage (about 0.1V) and doesn’t need a heatsink, even with maximum input voltage and at the full load current of 1.5A. Because it dissipates a lot less heat, that also means that less of the input supply power is wasted. Plus it has an electronic shut-down feature, allowing siliconchip.com.au a micro or other logic circuit to turn it off if necessary. In this “sleep” mode, it draws very little current. The only real disadvantage of a switchmode regulator (besides the extra complexity) is the high-frequency ripple on the output due to the switching action. But since the AP5002 operates at such a high frequency (typically 500kHz), the ripple has a low amplitude and sub-harmonics are not audible. It can be reduced even further by an external LC filter, to suit a particular application. SWITCH S1 INDUCTOR L1 + + iL PATH 1 VIN DIODE D1 PATH 2 C1 VOUT LOAD Fig.1: basic scheme for a switchmode buck converter. Voltage regulation is achieved by rapidly switching S1 and varying its duty cycle. The current flows via path 1 when S1 is closed and path 2 when it is open. In a practical circuit, S1 is replaced by a switching transistor or a Mosfet. Regulation So why do you need a regulator anyway? Well, there are a number of reasons. If you have a device which must run at a particular voltage (eg, 5V ± 0.5V or 4.5-5.5V), then you could just use a regulated plugpack or bench supply. However, depending on the length and thickness of the supply leads and the unit’s current consumption, there will be a voltage drop before the power reaches the device. Even if the supply is putting out exactly 5V, it’s possible that it may be below the minimum (in this case, 4.5V) by the time it reaches the unit. What’s worse, as the unit’s current draw changes, so will the voltage it receives, due to the cable drop and the output impedance of the power supply itself. Local regulation solves this problem. By placing a regulator board in close proximity to the device being powered and feeding a higher voltage to it, changes in the power supply’s output voltage become irrelevant. Also, there are times when (for various reasons) you want to use a linear power supply, eg, a mains transformer with its output rectified and filtered. Not only can the output voltage of this type of supply vary quite a bit with load but there is also 50/100Hz voltage ripple, due to the fact that the filter capacitor(s) charge and discharge over each mains cycle. This can cause hum in audio equipment and various other problems. An efficient switchmode regulator can turn this rather variable output into a nice, stable supply with a minimum of energy being wasted as heat. switchmode regulator works. Fig.1 shows the basic circuit. It uses a switch (in practice, a switching transistor or a Mosfet) to rapidly connect and disconnect the incoming power supply to the input end of inductor L1. The other end of the inductor connects to filter capacitor C1 (which acts as an energy storage device) and the load. As shown by the blue line labelled “PATH 1”, when the switch is closed, current flows through the inductor and then the load. The rate of current flow ramps up linearly as the inductor’s magnetic field strength builds. Then, when the switch opens, the current flow from the input supply is interrupted and the inductor’s magnetic field begins to collapse. This continues driving current into the output but at a diminishing rate. While the switch is open, the output current is sourced from ground, via diode D1 (the red line shown as “PATH 2”). In practice, because this current then flows to ground after passing through the load, it actually travels in a loop, through D1, L1, the load and then around again until either the inductor’s magnetic field is fully discharged or switch S1 closes again. By varying the switch on/off ratio, the average current through the inductor can be controlled and this, in combination with the load characteristics, determines the output voltage. The ratio of the switch on-time to the switching period (on-time plus off-time) is known as the duty cycle. However, because the inductor and load properties can vary, for a constant output voltage we can’t use a fixed duty cycle. Instead, the output voltage is monitored and if it is too low, the duty cycle is increased. Conversely, if the output voltage is too high, the duty cycle is decreased. This negative feedback provides the required regulation. There’s a bit more to it than that but in practical circuits, most of the details are taken care of by a switchmode IC. Circuit description We decided to use an AP5002 after surveying the range of switchmode Specifications Input voltage ......................................... 3.6 to 20V (absolute maximum 22V) Output voltage ................................... 1.2-20V (must be below input voltage) Dropout voltage ...............................................................typically 0.1V at 1A Output current ........................................................................... at least 1.5A Efficiency .............................................. can exceed 90%, typically over 85% Switching frequency ...................................................approximately 500kHz Quiescent current............................................ 3mA (10µA when shut down) Load regulation .....................................................................~1%, 1.5A step Line regulation ............................................................................ ~2%, 4-20V Switchmode basics Output ripple ................................................. <5mV RMS at 1.5A (see Fig.2) Before going further, let’s take a look at how a step-down (or “buck”) Transient response ..........~250mV overshoot, ~100mV undershoot, 1A step siliconchip.com.au February 2012  65 +IN Q1 IRF9333 CON1 1 S1A S1B –IN 4 S D K 2 G 3 4 A ZD1 15V 22 F 25V X7R 100nF 25V C0G/NP0 2 3 EN Comp 100k 1k 100nF 2 25V X7R OUTPUT OUTPUT 100k CON3 1 SHUT DOWN Vcc Vss 8 K FB 1 A D1 1N5822 2012 VR1 50k 22 F 100nF 4.7nF 2 1k MKT 100 F 25V 25V X7R 25V C0G/NP0 3 LOW ESR 1.8k –OUT LED+ 4 LED– 1nF 50V ZD1 A SC  CON2 1 +OUT 6 IC1 AP5002 Vss 7 L1 47 H 3A+ 5 MINISWITCHER 1.2-20V REGULATOR 1N5822 A AP5002 IRF9333 K K D DD D S SS G 8 4 1 Fig.2: the complete switching regulator circuit. Mosfet Q1 provides input reverse polarity protection while IC1 does the switching and regulation via negative feedback. Inductor L1 filters the output in combination with three capacitors across the ouput rail, while trimpot VR1 provides output voltage adjustment. regulator ICs available. This device has a good range of features and is low in cost. Fig.2 shows the circuit details. It’s based on the data sheet but with several important changes. As well as the switchmode regulator (IC1), you should recognise inductor L1 and Schottky diode D1 from the explanatory diagram (Fig.1). While the recommended inductor value is 10-22µH, we found that 47µH provides better duty cycle stability over a range of input and output voltages and load currents. It’s also a more common value and it provides better ripple filtering than a lower value inductor. Both the input and output lines are filtered using low-value (100nF) and high value (22µF) ceramic capacitors in parallel. This results in a very low ESR (equivalent series resistance) across a wide range of frequencies, reducing the current spikes in the input and output wiring. Note that the 100nF capacitors are specified with a ceramic C0G dielectric, as this provides the best performance over the widest range of frequencies and temperatures. Trimpot VR1 allows the output voltage to be adjusted. It forms part of a resistive voltage divider which is in the feedback path from the output to IC1’s FB (feedback) input at pin 1. IC1’s negative feedback keeps its FB pin at around 0.8V. This means that in order to get a 5V output (for example), VR1 is set to around 9.45kΩ. In practice, you just turn VR1 until the desired output voltage is achieved. 66  Silicon Chip VR1 is in the upper half of the feedback divider, with a 1.8kΩ resistor in the lower half, as this provides a more linear and progressive adjustment. However, there are advantages to using the opposite configuration (ie, with VR1 between FB and ground), the primary one being that if VR1 goes open circuit, the output voltage goes down rather than up. But then it’s trickier to set the desired voltage. The 4.7nF capacitor across VR1 is a “feed-forward” capacitor which reduces the gain of the feedback system to unity at high frequencies. This improves the circuit’s stability, like the capacitor across the feedback resistor often seen in op amp circuits. The 1nF capacitor and 1kΩ resistor in series between pins 1 (FB) and 3 (Comp) of IC1 also work to improve the loop stability of the regulator. These components provide frequency compensation, hence the labelling of pin 3. Pin 1 connects to the input of IC1’s internal error amplifier while pin 3 connects to the output and so these components are in the feedback loop and limit the slew rate of the error amplifier output. Pin 2 of IC1, labelled “EN”, is the enable input. If this is pulled low, the regulator shuts down – its internal switch turns off, the output pins go high impedance and its quiescent current drops to 10µA. A 100kΩ pull-up resistor to Vcc enables the regulator by default, while a 100nF capacitor filters the voltage at this pin to prevent the EN pin from rapidly toggling due to EMI (electromagnetic interference). EN can be driven low for shut-down and simply pulled high (via a resistor) for normal operation. Alternatively, it can be actively driven high and low. However, if actively driven high (not used here), the high voltage must be below Vcc. It’s also a good idea to drive the EN pin via a series resistor of about 2.2kΩ, to protect IC1. The input supply is normally connected to terminals 1 (positive) and 4 (negative) of CON1. A power switch can then be connected between terminals 2 and 3. If you don’t want a power switch, you can simply connect a short piece of wire (eg, 1mm tinned copper wire) between terminals 2 and 3. Alternatively, the positive input supply can be connected directly to terminal 3. P-channel Mosfet Q1 (a surfacemount type) protects IC1 against accidental reversal of the supply voltage polarity. This is a logic-level device with a very low on-resistance, so it can operate down to the minimum supply voltage for IC1 (3.6V), In addition, during normal operation, very little power is lost in Q1. Its on-threshold is typically 1.8V (maximum 2.4V), so by 3.6V its channel resistance is already quite low – around 33mΩ at 4.5V and 20mΩ at 10V and above. If the input supply voltage has the correct polarity, Q1’s gate is pulled below its source, which is initially no more than one diode drop below its drain. This is connected to the positive supply lead (clamped by the parasitic body diode). Since Q1 is a P-channel type, this turns it on. Its maximum siliconchip.com.au Parts List 1 PCB, code 18102121, 49.5 x 34mm (available from SILICON CHIP) 1 47µH 3A inductor (L1) (Altronics L6517) 1 50kΩ mini horizontal trimpot 4 2-way terminal blocks, 5mm or 5.08mm pitch (CON1, CON2) 1 2-way polarised header (CON3) 3 M3 x 6mm machine screws 3 M3 x 12mm tapped spacers Semiconductors 1 AP5002SG-13 switchmode regulator [SOIC-8] (IC1) (Element14 1825351) 1 IRF9333 P-channel Mosfet [SOIC-8] (Q1) (Element14 1831077) 1 1N5822 3A Schottky diode (D1) 1 15V 400mW/1W zener diode (ZD1) Fig.3: this shows the operation of the unit with 13V in and 7V out at 1.5A. The yellow trace is the voltage at the output pins of IC1 while the mauve trace shows the voltage across the load. The spikes in the latter trace corresponding with the output transitions are due to inductance in the scope probe ground lead. If you ignore that, there’s only a few millivolts of ripple at the regulator output. gate-source voltage rating is 20V, so zener diode ZD1 limits this to around 15V (for higher supply voltages). However, if the supply voltage is reversed, Q1’s gate is instead pulled above its source and so Q1 is off. The parasitic body diode is now reversebiased, so no current can flow into the circuit. ZD1 clamps the gate to no more than one diode drop above the source, with some current flowing through the 100kΩ resistor (up to a maximum of 0.22mA at 22V). With a correctly polarised supply voltage above 15V, ZD1 conducts and a small amount of the supply current passes through Q1’s 100kΩ gate resistor. This is no more than about 70µA at the maximum allowable supply voltage (22V). Below 15V, Q1’s gate has a very high resistance and so once its gate capacitance has charged up and Q1 is on, only a tiny current flows. The output voltage is available from terminals 1 & 2 of CON2. A LED can be connected between terminals 3 and 4, to indicate when the regulator is operating. The specified 1kΩ current-limiting resistor will suit some combinations of output voltage with some standard LEDs but may need to be reduced for other combinations (ie, siliconchip.com.au lower output voltages and/or blue or white LEDs). Transient response The 100µF electrolytic capacitor in parallel with the output filter has been added to improve transient response. If the regulator’s load suddenly drops (ie, its impedance increases), the output isn’t immediately reduced to compensate. This is partly due to energy stored in the inductor and partly due to the frequency compensation scheme required for stable operation. The result is a temporary spike in the output voltage. By increasing the output capacitance, we reduce the amplitude of this spike. With the circuit as shown, we measured an overshoot of around 0.25V with a step of over 1A. The undershoot when the load impedance suddenly drops (ie, current demand increases) is much lower, at less than 0.1V. These figures should be acceptable in most applications and will be reduced further by any input capacitance associated with the load – typically several hundred microfarads. Note that we have specified a lowESR type for the 100µF filter capacitor so that it has sufficient ripple current Capacitors 1 100µF 25V low-ESR electrolytic 2 22µF 25V X7R ceramic [4832/1812] (Element14 1843167) 3 100nF 25V NP0/C0G ceramic [3216/1206] (Element14 8820210) 1 4.7nF MKT 1 1nF 50V NP0/C0G ceramic [3216/1206] (Element14 1414710) Resistors (0.25W, 1%) 2 100kΩ 1 1.8kΩ 2 1kΩ capability. These are also usually rated for 105°C operation. Capacitors this small are usually only rated for around 500mA ripple current but in this regulator, the ripple is quite low and so heating isn’t a problem. In operation, the electrolytic capacitor is normally only heated to about 10°C above ambient (tested at 1.5A). Construction The MiniSwitcher is built on a PCB coded 18102121 and measuring 49.5 x 34mm. This has been designed as a double-sided PCB with some platedthrough holes and the top layer acting as a ground plane to reduce electromagnetic interference (EMI). Fig.4 shows where the various parts go. Begin the assembly by installing IC1 on the underside of the PCB. This February 2012  67 – LED + CON2 3A+ + 100nF 22 F IC1 Q1 1 22 F 100nF SHUTDOWN GROUND 5822 4.7nF D1 100nF + 1k 1 VR1 1.8k –IN CON3 1nF L1 47 H 100k 1k 15V ZD1 50k 100k SWITCH CON1 +IN If you don’t get it perfectly positioned on the first attempt, just reheat the solder and adjust it slightly. That done, solder the other pad, then go back to the first one and apply a little fresh solder, to reflow it and form a proper joint. The two larger 22µF ceramic capacitors can then be installed using the same procedure. – OUT + Through-hole parts 100 F TOPSIDE VIEW UNDERSIDE VIEW You can now proceed to install the through-hole parts, starting with the resistors. Check the values with a DMM before installing them, then fit diode D1 and zener diode ZD1, taking care to orientate them correctly. Follow with trimpot VR1, the 4.7nF MKT capacitor and then the terminal blocks. Be sure to dovetail the 2-way terminal blocks together (to make 4-way blocks) before pushing them down fully onto the PCB and soldering their pins. Make sure that their wire entry holes face towards the adjacent edge of the PCB. Note that there is provision on the board for the load and/or LED to be connected via a polarised header instead of a terminal block. This could be useful for loads drawing under 1A, such as computer fans. If you decide to install polarised headers instead, check the polarity of the fan plug and orientate them accordingly. You can mix and match 2-way terminal blocks and polarised headers if you like. The polarised header for the shutdown feature can then be fitted at bottom left. Orientate it as shown on the overlay diagram (Fig.4). The 100µF electrolytic capacitor can then be installed, followed by inductor L1 (47µH). The assembly can now be completed by fitting three M3 x 12mm tapped spacers to the corner mounting holes. Fig.4: the regulator is built on a small double-sided board and utilises both surface-mount and through-hole components. The top layer is a ground plane, minimising the current loops and thus keeping electromagnetic radiation outside the board to a low level. These top and bottom same-size views show the fully-assembled PCB. You will need a soldering iron with a fine conical tip to solder in the surfacemount parts. Unwanted solder bridges can be removed using solder wick. is in a surface-mount 8-pin SOIC package and its pins are sufficiently spaced for it to be soldered with a regular iron. First, check that it is orientated correctly, with its pin 1 towards the bottom edge of the board. That done, line its pins up with the pads and solder them in place. If your IC doesn’t have a dot to indicate pin 1, check to see whether it has a bevelled edge, as shown on Fig.4. Because its output and ground pins connect directly to its internal Mosfet switch, these are soldered to two large pads for better heat dissipation. The other four pins connect to individual pads as usual. Use fresh solder and ensure it has been heated enough to flow properly. If you don’t do this, it’s possible for solder to adhere to one of the pins but not actually flow under the pin and onto the associated pad. Install Mosfet Q1 next, using the same technique. It too has large pads for its multiple drain and source pins. Be careful because its orientation is opposite to IC1, ie, its pin 1 goes towards the top of the board. Now check IC1 and Q1 for any unwanted solder bridges between adjacent pins (ie, ignore those between pins that solder to the same pad). If you do find any, they can be easily removed using solder wick (or desoldering braid). The 100nF and 1nF ceramic capacitors in the 3216/1206 packages are next on the list. The easiest way to install these is to first melt some solder onto one of the pads. You then hold the capacitor alongside this pad using tweezers, reheat the solder and slide the capacitor into place. Setting up & testing The first step is to turn VR1 fully anti-clockwise, then back it off a little. That done, connect a power supply between terminals 3 & 4 of CON1 (eg, a 12V plugpack or a bench supply). The Table 1: Resistor Colour Codes o o o o No.   2   1   2 68  Silicon Chip Value 100kΩ 1.8kΩ 1kΩ 4-Band Code (1%) brown black yellow brown brown grey red brown brown black red brown 5-Band Code (1%) brown black black orange brown brown grey black brown brown brown black black brown brown siliconchip.com.au This photo of the MiniReg linear regulator (December 2011) shows just how inefficient it is compared to the MiniSwitcher. This is the size of heatsink it requires in order to handle a current of 1A if there is a large voltage differential between input and output (eg, 14.4V input and 5V output). By contrast, the MiniSwitcher can handle currents up to 1.5A and doesn’t require a heatsink at all, regardless of the input-to-output difference. positive lead should go to terminal 3. It’s also a good idea to connect a DMM set to measure current in series with the supply, if possible. You may also want to connect a LED across terminals 3 & 4 of CON2, with the anode (longer lead) to terminal 3. Depending on the output voltage and LED colour, it will be driven at 1-20mA. If the LED is too dim (eg, at low output voltages), use a lower value resistor and if it is too bright, increase the value. For output voltages of 5V and below, it’s probably a good idea to change this resistor to 300-470Ω, while for output voltages above 12V, you may want to increase it to, say, 2.2kΩ. Note, however, that the LED will not light if the regulator’s output voltage is lower than the LED’s forward voltage (1.8V for a red LED and 3.3V for a blue LED). If you want to use a 12V LED (ie, one with a built-in resistor) and the output voltage is no more than say 15V, replace the 1kΩ resistor with a wire link. Alternatively, the LED can simply be connected across the output terminals, in parallel with the load. Now apply power and check that the current quickly drops to just a few milliamps. Assuming it does, check the voltage at the output, ie, between pins 1 & 2 of CON2. This should be around 1V, depending on the exact position of VR1. If this is correct, turn VR1 and check that this adjusts the output voltage. Note that you may hear some whine from the inductor if you set it below 1.2V, as this typically results in some sub-harmonic oscillation. Assuming all is well, adjust VR1 to give the desired output voltage. It’s a good idea to make the final adjustment later, with the power supply you will be using in your application (assuming it’s different from the one you’re using for the set-up). If you have a low-value, high-power resistor (eg, 4-10Ω 10W), connect it across the output terminals and check that the set voltage is maintained. This assumes that with your set voltage, the current draw will be within the permissible range (up to 1.5A) and that your test supply can deliver enough power to the regulator. Troubleshooting If the board isn’t working, switch off and check the solder joints with a magnifying glass. In particular, check IC1 and Q1 carefully, as it isn’t always obvious when the solder has adhered to a pin and not to the pad. Assuming there are no soldering problems, the other likely cause of a fault is an incorrectly orientated component or a part installed in the wrong location or having the wrong value. If all is well, install the regulator board into the chassis you want to use it in and monitor the output voltage while making the final adjustment to VR1. You can then use a dab of silicone sealant or hot-melt glue to prevent it from being changed accidentally. SC :HWKR :HWKRXJKWZH¶GUXQRXWEXWWKHUHRQWKHEDFNRIWKHVKHOI LAST FEW: $AVE $$$ TV ACROSS AUSTRALIA Your easy reference guide to TV reception across Australia Buy direct from SILICON CHIP bookshop Travelling around Oz? Want to know where to aim your antenna? This book will tell you! RRP: Lists channels, location and polarity of all analog transmitters and translators (digital services are usually co-sited). A MUST-HAVE with loads of other TV-related data too! Even if you aren’t travelling, this is highly useful in STRICTLY FIRST COME, troubleshooting local TV reception problems. FIRST SERVED. VERY LIMITED STOCKS LEFT! All this information in one handy source! $ siliconchip.com.au 39 95 ONLY while stocks last: 29 $ 95 +p&p SEE P98 for handy order form February 2012  69