Fullscreen HTML5Med Res (??MB)HTML4

Simple Voltage Inverter Doubler This simple and low-cost circuit can produce a voltage around twice its DC input, or instead, a negative voltage of similar magnitude to the input. That can be handy in many situations, such as running op amps from a battery or DC supply, driving Mosfet gates, or providing a wider output range for adjustable regulators. by John Clarke I f you are building a project and the power supply voltage is insufficient to drive some components, or you need to derive a negative supply from the positive supply, this little project can be the answer. It is based around a 555 timer, a few diodes, resistors and capacitors on a reasonably compact printed circuit board (PCB). The circuit acts as a voltage inverter or almost doubler, depending on how you build it. It can deliver an output of a few tens of milliamps. A voltage inverter can be very useful for many applications. Suppose you need to use an op amp for processing audio. A negative supply can make the circuit easier to design with fewer parts as the audio signal can be ground-referenced. Without the negative supply, the audio signal would need to be raised to around half the positive supply and coupled with capacitors. In some cases, using a split DC supply can mean insufficient headroom for signal processing, while using the negative supply almost doubles the op amp input and output swings. A voltage doubler can be helpful in 90 Silicon Chip many situations, for example, if you need to bias an N-channel Mosfet gate above the positive supply to use it as a high-side switch, or to power a small 24V DC relay from a 12V DC supply. Note that there are some losses in the circuit. As a result, when used as a ‘doubler’, the actual output will be around 3-3.5V less than double the input voltage. Similarly, when used as an inverter, the resulting negative voltage is a couple of volts less in magnitude than the positive input. Most of the voltage losses are from the 555 IC for both doubling and inversion, as its output does not go entirely to the positive supply when under load. There are also voltage drops across the diodes. But if you are prepared to accept these losses, the circuit can be useful. The output current is up to about 30mA, although more is available with higher input voltages. Performance Figs.1 & 2 are plots of output current and voltage against input voltage. They should allow you to decide whether the circuit suits your application. The current versus Vout graphs Australia's electronics magazine Features & Specifications ▬ Operates from 9-15V DC (Vin) ▬ Produces either a ‘doubled’ or ‘inverted’ DC output ▬ ‘Inverted’ output voltage is about -(Vin − 3V) (see Fig.1) ▬ ‘Doubled’ output voltage is about Vin x 2 − 3.5V (see Fig.2) ▬ Output current up to about 30mA (see Figs.1 & 2) ▬ compact PCB (37 x 42.5mm) ▬ Inexpensive and few parts required (555 timer plus a few diodes, capacitors and resistors) are shown only for 9V, 12V and 15V supply inputs; below 9V, the output is possibly too low to be useful. The input voltages are the voltage applied to the 555 timer, which is not necessarily the same as at the Vin terminal. If you want a voltage doubler or inverter that runs from 1.5-5.5V, see the text under the “Alternatives” heading for ICs that can do that efficiently. We created this circuit because we needed a negative voltage to revise our 30V 2A Bench Power Supply, originally published in the October & November 2022 issues. We’re changing it because the mains transformer it used is now unavailable, and the new transformer does not have a tap for us to derive the -8V supply like the original design. So, we use this circuit as a voltage inverter to deliver the required -8V from the +12V DC rail. The inverter is ideal since we only need about 13mA at between -9V to -8V. That’s within its capabilities. The circuit was designed to be simple and use commonly available parts. Because of its simplicity, it can easily be configured to provide either voltage inversion or doubling. Circuit details Fig.3 shows the circuit for the Voltage Inverter/Doubler, or VI/D for short, with two output options to implement the doubler and inverter functions. Much of the circuitry is common for both versions, including the 555 timer and its associated timing components. The incoming supply comes from the Vin and the GND terminals. From Vin, the supply passes through either diode D3 or resistor R1. D3 is to prevent damage should the incoming supply polarity be reversed. If you siliconchip.com.au are permanently connecting the VI/D to the incoming supply, you could bypass D3 with a wire link so that there is more available output at Vout. When using D3 or the wire link, zener diode ZD1 and R1 are not installed. The 555 timer (IC1) supply cannot exceed 16V. If the upstream supply can be higher than that, or you wish to set Vout to a particular level, then R1 and ZD1 should be installed instead of D3 or a wire link. ZD1 and R1 provide voltage limiting for the VI/D supply. The zener diode limits the voltage, while R1 limits the current through the zener to a safe level. These component values depend on your application; we will provide examples later. Figs.1 & 2: plots of the output current and voltage against the input voltage for the Voltage Inverter (left) and Voltage Doubler (right). Oscillator IC1 is connected to run as an oscillator with a duty cycle close to 50%. Pin 3 provides a square wave output, and the 1nF capacitor, 47kW resistor and 4.7kW resistor at pins 2 and 6 set the frequency and duty cycle. The 1nF capacitor is charged via 4.7kW and 47kW resistors from the positive supply. While it’s charging, output pin 3 of IC1 is high (near the positive supply). When the capacitor voltage reaches 2/3 of the supply voltage (as detected by the pin 6 threshold input), pin 7 (the discharge output) goes low, as does the pin 3 output. With pin 7 low, the capacitor is discharged via the 47kW resistor until its voltage reaches 1/3 of the supply, as detected by the trigger input at pin 2. Now the pin 3 output goes high again, and the pin 7 pin goes high-­ impedance, allowing the capacitor to recharge. The process repeats continuously. As the capacitor is charged via the 47kW and 4.7kW resistors (a total of 51.7kW) and discharged via the 47kW resistor, you can expect the output to be high for a little longer than it is low. However, it is close enough to 50% for this application. The oscillation frequency is 14kHz (1.44 ÷ [{4.7kW + 2 × 47kW} × 1nF]). The waveform can be seen in Scope 1, where the top yellow trace shows the capacitor voltage, and the lower cyan trace shows the 555’s pin 3 output. That was taken with the output (Vout) unloaded. The pin 3 output of IC1 drives the voltage doubler or inverter. Fig.4 siliconchip.com.au Fig.3: the circuit diagram for both the Inverter and Doubler. D3 is an optional component that prevents damage if the supply polarity is reversed, while R1 is only installed when D3 is not present. shows how the inverter section works, while Fig.5 explains the voltage doubler. For simplicity, the voltage drop across the diodes is shown as 0.7V, and the voltage sag at pin 3 of IC1 is ignored. Voltage inverter operation When IC1’s pin 3 is high, C1 charges to 0.7V less than the supply via D1 (left side of Fig.4). When pin 3 goes low, the positive side of C1 goes to 0V and the negative side goes negative. Note that the voltage across C1 does not change between the two halves of the diagram. C1 charges C2 via D2 to a negative voltage similar to the positive input Australia's electronics magazine Scope 1: the IC1 (555) timer waveform at pins 2 & 6 is shown in yellow, while the output (pin 3) is shown in cyan. The frequency is around 13.2kHz. September 2023  91 capacitors C1 and C2 are rated at 35V for voltage doubling. While C1 could be a lower-rated type, using 35V for both avoids confusion. Practicality Both the Voltage Doubler (top) and Inverter (bottom) modules only require a 555 timer IC and a handful of other components to build. voltage minus the 1.4V worth of diode drops; in this case, -7.6V (-1 × [9V – 1.4V]). Voltage doubler operation For the voltage doubler, diode D1 charges capacitor C1 to the supply voltage (minus 0.7V) when IC1’s pin 3 output is low (left side of Fig.5). If this is when power is first switched on, then the initially discharged capacitor C2 will charge about 1.4V below the supply via D1 and D2, shown as current i2. When IC1’s pin 3 goes high (right side of Fig.5), the negative side of C1 is lifted to the supply voltage, so the positive side of the capacitor will be close to twice the supply (9.0V × 2 − 0.7V). Note that the voltage across the capacitor is the same as before (8.3V). Diode D2 is forward-biased, and C1 charges C2, with another 0.7V loss. After a cycle or two, the voltage across C2 will be twice the supply voltage minus the 1.4V drop across D1 and D2. Since IC1 can be powered from up to 15V (the recommended maximum), 92 Silicon Chip As mentioned earlier, IC1’s pin 3 output does not swing fully to the positive supply rail or ground (0V) when under load. There is about a 2V drop at pin 3 when high and under load. The effect is that the output (Vout) does not reach the voltage expected. These losses also mean you will need at least a 9V supply to gain any reasonable voltage at the output. If the circuit doesn’t provide enough voltage for your application, you could use 1N5819 schottky diodes instead of D1, D2 and D3 (if D3 is used). That will give a little more output voltage due to their lower forward voltages. A CMOS equivalent to the 555 timer, such as the 7555 or LMC555, won’t improve the output voltage. While at very low load currents (less than 0.8mA), the outputs will swing reasonably close to the supply rails once there is a load, the voltages will drop substantially. You can simulate the 7555 pin 3 output with an 875W resistor in series when high and a 62.5W series resistor when low. We simulated the inverter in an LTspice file that you can download from the Silicon Chip website. If you want to test the doubler function, you can rearrange C1, C2, D1 and D2. The main problem with the simulation is that the 555 pin 3 output does not reproduce the actual voltage drop for the positive level output under load. Alternatives If you are after a voltage doubler at a higher output current, you may be interested in the Circuit Notebook entry “High-Current Voltage Doubler” by Dayle Edwards (September 2009; siliconchip.au/Article/1564). That circuit provides voltage doubling from an input of 5V, 6V, 9V or 12V with an output current of up to 1.5A. Specialised ICs are also available, although they usually have somewhat limited input voltage ranges. For example, the Intersil ICL7660 (1.5-10V), ICL7660A (1.5-12V) and ICL7662 (4.5-20V) are all capable of operating as voltage doublers or inverters. They are all still available (although the 7662 is only made by AD/Maxim now). For an efficient voltage inverter that can run from 1.5V to 5.5V with a 25mA output current, consider the Analog Devices ADM8828 IC, especially for inverting the voltage from a USB supply. Similarly, the LM2662 is suitable as an inverter or doubler at up to 200mA output and can also operate from 1.5V to 5.5V. Other similar chips are on the market; we can’t list them all here. Diode D3 vs zener diode ZD1 As mentioned earlier, ZD1 and R1 can be installed instead of D3 if the supply voltage could exceed 15V. ZD1 can be selected between 9.1V and 15V, depending on your required output voltage. You will then need to calculate an appropriate value for resistor R1. For example, say you want to use the VI/D as an inverter delivering around Fig.4: the two phases of the Inverter charge pump. Fig.5: the two phases of the Doubler charger pump. Australia's electronics magazine siliconchip.com.au -9V at up to 13mA. Fig.1 shows that the circuit needs to be supplied with 12V to obtain this voltage at the output at the required current. Therefore, you can select a 12V 1W zener diode for ZD1. The value of R1 will then depend on the expected supply voltage. For example, if Vin is 21V, the voltage across R1 will be 21V − 12V or 9V. A 12V 1W zener diode’s maximum current is 83.33mA (1W ÷ 12V). Typically, the zener should be used with at least a 50% power derating to prevent overheating. Also, the minimum current through the zener diode should be about 5mA to maintain voltage regulation. So the zener diode current range of operation should be 5mA to 41.6mA. The value for R1 is Vin minus the zener voltage (12V), then divided by the 50% power derating current of 41.6mA. This gives 216W, so a 220W resistor can be used. Its dissipation will be V2 ÷ R1, ie, 368mW (9V2 ÷ 220W). A 1W resistor is thus ideal; a 1/2W or 0.6W resistor could be used, but it would run hot. We can draw up to about 36.6mA (41.6mA – 5mA) before the zener current drops to 5mA. If we want 13mA at Vout, assuming 75% efficiency for the converter (which is about right), the input current will be 17.3mA (13mA ÷ 75%). That means some 17.7mA remains flowing through ZD1, more than enough to maintain regulation. There is also sufficient current headroom to allow for the current drawn by the oscillator, around 5mA. Construction The circuit is built on a PCB coded 04107222 that measures 37 × 42.5mm. The orientation and positions for D1, D2, C1 and C2 for the inverting version Parts List – Voltage Inverter / Doubler 1 double-sided plated through PCB coded 04107222, 37 × 42.5mm 1 NE555P timer or equivalent, DIP-8 (IC1) 2 1N4004 400V 1A diodes (D1, D2) 1 1N4004 400V 1A diode (D3; optional – see text; not used for Supply) 1 1W zener diode (ZD1; optional – see text; 12V for Bench Supply) 1 100μF 16V radial electrolytic capacitor 1 100nF 100V MKT polyester capacitor 1 1nF 100V MKT polyester capacitor 1 47kW ¼W 1% metal film axial resistor 1 4.7kW ¼W 1% metal film axial resistor 1 1W axial resistor (R1; optional – see text; 220W for Bench Supply) Additional parts 2 100μF 35V radial electrolytic capacitors (C1, C2 – for voltage doubler) 2 100μF 16V radial electrolytic capacitors (C1, C2 – for voltage inverter) are shown on the top of the PCB. For the doubler version, they are on the underside of the PCB instead. These positions are shown in Fig.6. Note that only the inverter is shown with the different options for D3 and ZD1/R1 in Fig.6, but you could also use ZD1/R1 with the doubler. You would just leave off D3 and fit ZD1/ R1 instead. The components are intended to be installed on the top side of the PCB for all versions. The screen printing was placed on the underside for the doubler components to avoid clashing with the inverter markings on the top side. There are four mounting points on the PCB for standoffs. The PCB can also be mounted vertically using stiff tinned wire at the Vin, GND and Vout terminals. An extra pad is provided at the top of the PCB for extra mechanical support if required in such an application. As mentioned, diode D3 is installed for reverse polarity protection if required or replaced with a wire link if not required. Alternatively, if input Fig.6: the PCB overlay for the Inverter or Doubler project. While the Doubler version’s silkscreen is on the underside of the PCB, the components are installed on the top side of the PCB. siliconchip.com.au Australia's electronics magazine supply regulation is needed to obtain a particular output voltage or to limit the supply voltage to IC1, R1 and ZD1 should be installed instead of D3 or the wire link. Begin construction by fitting the axial components for the version you require (resistors and diodes). Ensure the diodes are orientated as shown, with all their cathode stripes towards the top of the PCB. IC1 can be soldered directly to the PCB, ensuring it has the correct orientation. Follow with the smaller MKT capacitors, which are not polarised. The three electrolytic capacitors have space to lie flat onto the PCB, although you could mount them vertically if desired. Pay close attention to their orientations as they are reversed between the inverter and doubler configurations! In all cases, the striped end is negative, which is also the side with the shorter lead. Testing There isn’t much to it; apply a voltage to the input that’s close to what you’re using in the final application and check that the output is higher (for the doubler) or negative (for the inverter) and about the expected magnitude. Apply a load (eg, using a 5W resistor) and check that it doesn’t drop further than expected. If it doesn’t draw any current, draws too much current or the output voltage(s) are wrong, check that all the components are in the correct locations and of the right types as per whichever of Fig.6 matches your use case. Also check that the solder joints have formed properly and that there are no shorts between pads or component leads. SC September 2023  93