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Circuit Surgery Regular clinic by Ian Bell Diode power circuits and ideal diode controllers U ser Cuocomponents posted the following question on the EEWeb forum. ‘Can anyone suggest a device that realises the function of an ideal diode/OR-ing controller with the following parameters: input voltage range from 2.5V, Qualification: AEC-Q100.’ Other members replied with a couple of device selections, which was what Cuocomponents was looking for. However, inspired by this question, we will look at ideal diode controllers and related circuits in more depth. If you have not heard of these circuits, the headline description is that they are used in power supply circuits for reverse-voltage protection and switching between power sources (eg, batteries and mains DC adapters). Discrete silicon and Schottky diodes can be used in these circuits, but ideal diode controllers provide improved efficiency and performance. The AEC-Q100 referred to in the question is a stress-test qualification for packaged integrated circuits used in automotive applications. Our discussion will introduce ideal diode controllers after covering the more basic circuits they replace; we won’t look at details of the AEC-Q100 qualification. Supply reversal protection With battery-powered equipment there is always the risk that the user will insert or connect the battery the wrong way round. Unfortunately, circuits may be damaged or destroyed by supply reversal, so steps should be taken to avoid this. It is possible to try to prevent battery reversal by mechanical means. For example, the commonly used PP3 9V battery has different terminal shapes for positive and negative. Despite this, it is still possible to touch the connector to the circuit the wrong way wrong and other batteries (such as AA, C and D types) may be easily inserted the wrong way in certain products. In some systems, for example automotive applications, transient reverse voltages may occur on the supply due to switching inductive loads, which may damage electronic circuits on the same supply. Electronic reverse supply voltage projection is therefore often needed and can be achieved straightforwardly using a diode, as shown in Fig.1. Practical Electronics | July | 2023 VF D1 + Battery IF VBat + VCir Circuit – – Fig.1. Battery-powered circuit with reverse-voltage protection diode. Simple diode protection in Fig.1) the diode will dissipate power (equal to VFIF) which will waste energy from the battery, reducing battery life. The diode also results in a lower voltage across the circuit (VCir) than the battery voltage (VBat), specifically VCir = VBat – VF. This can also reduce battery life by reducing the amount of battery charge that can be used – more on this later. Both these problems can be reduced if the forward voltage can be reduced. One way to do this is to use a Schottky diode. In the circuit in Fig.1, the battery is shown connected correctly. The diode is forward biased and current flows from the battery Schottky diode protection to the circuit, which will operate normally. Schottky diodes are formed with a metalIf the battery is reversed the diode becomes semiconductor junction, unlike the reverse biased and will not conduct. There junctions of P and N-type silicon used in will be a small leakage current, but this standard silicon diodes, such as the devices will not damage the circuit. mentioned above. The typical forward Various things need be to be considered voltage for a Schottky diode is 200mV to when using the circuit in Fig.1. A diode 300mV, rather than the 600mV to 700mV must be selected with a maximum forward for silicon PN junction diodes. These typical current rating above the supply current values are approximate, and relatively low of the circuit (IF is the forward current and high currents will produce voltage drops outside these ranges, but under in Fig.1). The diode’s maximum reverse comparable conditions Schottky diodes voltage must be larger than the battery have a significantly smaller forward voltage. voltage, although this is unlikely to be We can see the difference between a problem with typical battery-powered Schottky and silicon diodes if we look circuits and rectifier didoes. For example, at their current against voltage (IV) the common 1N4001 has a reverse characteristics. From these curves we breakdown (blocking) voltage of about can find the voltage drop at a given current 50V and other devices in the 1N400x range and hence find the power dissipation have much higher values. The 1N400x in the reverse protection diode when a range have a maximum forward current circuit is operating at a particular supply of 1A and higher current rectifier diodes current. We can plot IV characteristic of are of course available (eg, the 1N540x diodes using a DC sweep in LTspice – for range at 3A). For circuits with smaller example, using the setup shown in Fig.2, supply currents, signal diodes such as the 1N4148 (300mA maximum forward current), could be used. Signal diodes are not available with such high breakdown voltages as rectifier diodes, but for most battery circuits this will not be an issue. Adding a reverse protection diode may cause problems. Due to the forward voltage across it (VF Fig.2. LTspice circuit to obtain diode IV characteristics. 51 Fig.3. Results of the simulation shown in Fig.2. between a resistance M1 of 1MΩ below 0V + + and 10mΩ above 0V, but is sufficient Battery Circuit VCir VBat to compare the shape – – of a close-to-ideal diode on the IV plots of the real diodes. Fig.5. Battery-powered circuit with In comparison, the reverse-voltage protection using models of the 1N4001 P-channel MOSFET. and 1N5817 are based on diode physics. Fig.3 shows the results from the simulation in Fig.2. The plots are configured to cover the same current range, up to 1.0A (the limit for the two real diodes). The plots show that, as expected, the Schottky diode forward voltage is lower than the silicon diode – at around 0.20.4V rather than 0.8-1.0V. This is slightly higher than the ‘typical’ values (eg, 0.6 to 0.7V for silicon diodes) due to the relatively high current. The ideal diode conducts above 0V, with the trace showing an almost vertical line due to the very low on-resistance. For comparing diodes used for reverse power protection we can assume a certain supply current from the circuit and look at the diode voltage drops at this current. Fig.3 has cursors placed in the real diode traces at a current of 500mA. In LTspice these voltages can be read as 909mV for the 1N4001 and 369mV for the 1N5817 (these values can also be read more approximately from Fig.3). The ideal diode current can similarly be read using LTspice, but given that this is a linear piecewise model switching to RON at 0V, it is also given by IFRON = 500mA × 0.01Ω = 5mV. Using VFIF we find the power dissipation to be 454mW for the silicon diode; 185mW for the Schottky diode; and 2.5mW for idealised diode. Using a Schottky diode more than halves the power wasted in the diode in this example. Of course, the power dissipated in the idealised diode is much lower. In addition to simply wasting power, designs that involve high dissipation in a reverse protection diode may require a heatsink for the diode, increasing cost, size and complexity. Choice of diodes (Schottky or silicon) should not only be determined by the continuous supply current for the circuit, but also the potentially large current that may flow at switch-on (inrush current), particularly if the circuit has large capacitors across the power supply that have to charge via the diode. Battery considerations We mentioned above how the use of a reverse protection diode can reduce effective battery life. Consider a rough example to illustrate this. Say we have a circuit operating from two AA 1.5V alkaline batteries. Assume the circuit will Fig.4. AA alkaline battery characteristics from Duracell datasheet. which sweeps the voltage across all three diodes from –0.4V to 1.0V. Here we investigate the IV curves of the IN4001 silicon rectifier diode and the 1N5817 Schottky diode. The 1N5817 has a similar maximum forward current rating to the IN4001 (1A), but lower reverse voltage (20V). We also compare an idealised diode. The 1N5817 is included in LTspice’s library, but the 1N4001 is not, so we need to add a .model statement to the schematic (as shown in Fig.2). SPICE models for the 1N4001 are available online, including via the LTspice Wiki: https://bit.ly/pe-jul23-ltsp The ideal diode is modelled with a forward voltage of 0V and very small on-resistance of 10mΩ. This simple piecewise model just switches 52 Fig.6. LTspice circuit to compare Schottky diode and MOSFET reverse-voltage protection Practical Electronics | July | 2023 Fig.8. LTspice circuit from Fig.6 with capacitors across the circuit supply. Fig.7. Results of the simulation shown in Fig.6. work down to 2.2V and consumes 100mA. To understand what might happen we need to look at the discharge curve for the battery – these can be found on the manufacturer’s datasheet. As an example, Fig.4 shows the constant-current discharge characteristics for a Duracell Ultra Power MX1500 1.5V AA (LR6) Alkaline-Manganese Dioxide Battery. If we connect the batteries directly to the circuit, and assume that they both discharge exactly equally, then the circuit will function until the battery voltage reaches about 1.1V (total 2.2V), this occurs after about 23 hours and represents most of the available life of the batteries. If we use the 1N4001 as a reverse-protection diode the voltage drop is about 0.8V at 100mA, so each battery has to provide 1.1 + 0.8/2 = 1.5V for the circuit to function. This reduces the usable life to only 1 hour. With the 1N5817 Schottky diode the voltage drop is about 0.3V at 100mA, so each battery has to provide 1.1 + 0.3/2 = 1.25 V, which corresponds to a usable life of about 13 hours. The usable life points are shown by red marker dots on Fig.4. The Schottky diode is considerably better than the silicon diode in this example, but still has a large impact on usable battery life. As we noted earlier, the Schottky diodes will also dissipate less power so are usually the preferred option compared with silicon diodes. Schottky diodes are more expensive (for example the 1N4001 is about £0.10 and the 1N5817 is about £0.30 for one-offs from a major UK distributor at the time of writing). These costs are unlikely to be an issue for constructing individual circuits but may be more important for largerscale manufacture in some cases. Do bear in mind that a Schottky diode’s higher reverse leakage may be important in some applications. MOSFET protection It is possible to use MOSFETs in place of diodes for reverse-voltage protection. Fig.5 shows a version of the circuit in Fig.1 with the diode replaced by a P-channel MOSFET. The diode shown in Fig.5 is the body diode of the MOSFET, not a separate component – however, it is important that it is in the same direction as the protection diode in Fig.1. The body diode will conduct initially when power is switched on, allowing source voltage (at the circuit’s Practical Electronics | July | 2023 positive supply) to rise. Once the circuit voltage exceeds the MOSFET threshold voltage the transistor will switch on and conduct the current from the battery. Because this is a P-channel device the gate-source voltage must be negative with respect to the gate for conduction, which is what occurs in the situation shown in Fig.5. If the battery is reversed the gate-source voltage required for MOSFET conduction does not occur and supply current does not reach the circuit. A similar circuit with an N-channel MOSFET in the ground connection can also be used. The circuit in Fig.5 has the advantage that the voltage across the MOSFET can be significantly smaller than a Schottky diode in the same application. The MOSFET on-resistance can be in the tens of milliohms range or lower, resulting in low dissipation. An LTspice circuit to simulate the arrangement in Fig.5 is shown in Fig.6. The 1N5817 Schottky is included for comparison. The circuit is represented by a 6Ω resistor for both devices. The MOSFET used is the IRF7404, which was a somewhat arbitrary pick from the LTspice library. It is rated at 6.7A, which is much higher than the 500mA flowing in this example. Simulation applies a 3V supply in the correct orientation for the first 100µs and then reverses it to –3V. The results of the simulation in Fig.6 are shown in Fig.7. It can be seen that both the Schottky diode and MOSFET protect the circuit from the reverse-voltage (OutD and OutM are both zero after the input voltage reversal). While the correct polarity voltage is applied the forward-voltage drop across the Schottky diode of about 360mV is visible on the OutD trace. The OutM trace is very close to the 3V supply, with a measured drop of about 22mV. At 500mA this is a dissipation of 11mW, which is significantly smaller than for the 1N5817 at 500mA, as calculated above. The much lower voltage drop across the MOSFET would also lead to a close-to maximum available battery life in the scenario discussed above. Fig.8 shows a version of Fig.6 with capacitors added across the circuit power supply. This is to be expected, as many circuits will have decoupling capacitors, and in scenarios where transient supply reversals or shorts could occur, larger capacitors may be used to hold up the supply to the circuit during the transient, or provide time for systems such as those using microcontrollers to such down cleanly if appropriate. As with the other examples, the devices and values used in Fig.8 are for illustrative purposes and do not represent recommendations or a specific design solution. The results are shown in Fig.9. The voltages across the two circuits are shown in the top pane. It can be seen that the voltage across the circuit protected by the MOSFET (represented by the resistor R1) drops relatively quickly. This is because, unlike the diode, the MOSFET is able to conduct significantly in the reverse direction, returning energy from 53 Fig.11. LTspice schematic to illustrate diode-OR power source circuits. Fig.9. Results of the simulation shown in Fig.8. the capacitor to the battery – the reverse current pulse can be seen on the I(M1) waveform (MOSFET drain-source current). This lack of reverse-current protection from the MOSFET will be undesirable in some situations and significantly limits the possibilities of a capacitor being used to maintain the supply voltage during transient reversals. The diode has the disadvantage of higher voltage drop and power dissipation, but its reverse current is close to zero, so it protects against energy return to the battery, and the capacitor is better able to maintain the supply, although a small current will flow due to reverse leakage. diode-OR A circuit related to the reverse supply protection we have been discussing is the diode-OR circuit shown in Fig.10. There are two power supplies for the circuit. If only one supply is connected then its associated diode will conduct, and the Supply2 Supply1 VSupply2 D2 D1 + VSupply1 VCir Circuit – Fig.10. Diode-OR power source circuit. 54 Fig.12. (right) Results from the simulation in Fig.11. circuit will behave in the same way as the reverse-protection circuits discussed above. If both supplies are present, with one sufficiently higher than the other, then the diode for the supply with the higher voltage will be forward biased, and the other reverse biased. The higher supply will provide power to the load. If both supplies have the same, or very similar voltage then both diodes will conduct and both supplies will deliver power to the load (this is called load sharing). The diode-OR circuit is not restricted to two inputs, more supplies can be connected via diodes. This type of circuit has a number of applications, including providing a means for mains DC power adaptors to take over from internal batteries when they are plugged into a product, or switching from primary to backup batteries. Load sharing can in principle be implemented as noted, but may be difficult to achieve effectively in practice due to differences in individual diodes and thermal effects. Fig.11 shows an LTspice circuit to illustrate the principles of diode-OR power source circuits. Supply1 (V1) is steady a 2.5V while Supply2 (V2) ramps from 0V to 3.5V over one second. The resistor R1 is used to represent the circuit being powered. Schottky diodes are used for the reasons discussed above. The results are shown in Fig.12. The upper plane shows the voltages. The circuit voltage remains at about 2.15V (2.5V from Supply1 minus the 350mV forward voltage drop of D1) until the voltage on Supply2 gets close to 2.5 at around 0.7s. The circuit voltage then increases, following the increase in Supply2, staying at approximately 350mV below Supply2 due to the diode voltage drop. The lower pane in Fig.12 shows the currents. From 0.68 to 0.76s the two diodes both supply current to the circuit – load sharing is occurring. At around 0.714s Supply2 equals Supply1 at 2.5V and the diode currents are equal. Our discussion on reverse-voltage protection and diode-OR circuits has shown the usefulness of diodes in power supply Simulation files Most, but not every month, LTSpice is used to support descriptions and analysis in Circuit Surgery. The examples and files are available for download from the PE website. Practical Electronics | July | 2023 Supply1 M1 In Gate + Out Ideal diode controller Circuit Control Gnd Status – Fig.13. Ideal diode controller concept circuit (reverse protection). circuits, but has also highlighted some deficiencies in terms of power dissipation and the impact on supply voltage and hence Fig.15. LTspice circuit for LTC4412 used to control switchover between a battery and a battery life in battery-powered systems. We mains adapter (based on example file from Analogue Devices). have also seen that MOSFETs can be used in place of diodes to significantly reduce voltage drop and power dissipation. However, MOSFETs may conduct in the reverse direction in some cases, which can cause significant problems in some applications. Furthermore, the basic MOSFET circuit in Fig.5 may not work in all cases – for example, if the supply voltage is not correctly matched to the required gatesource voltage. For high voltages, a potential divider can be used to reduce the gate voltage, but if the supply voltage is too low the solution is more complex – for example, a charge pump can be used to obtain the voltage required to drive the gate. The diode-OR circuit could also make use of MOSFETs, but providing the gate control is not as straightforward as for simple reverse supply blocking. Improved protection designs All of the issues above can be addressed by using MOSFETs in place of the diodes with more sophisticated control of the MOSFET switching, along with other functionality such as charge pumps for gate drive voltages (if needed). There are a large range of integrated circuits from various manufacturers which provide this general capability, with different specific features and target applications. These chips are often referred Fig.16. Results from the simulation in Fig.15. to as ‘Ideal Diode Controllers’ because they control MOSFETs to perform like ideal diodes, although some devices are given more connected to the input supply and the circuit being powered specific names such as ‘Reverse Polarity Protection Controller’ and that allows them to sense what is happening and hence control the MOSFET gate appropriately. Their own power ‘Diode-OR Controller’. Fig.13 and Fig.14 show concepts of how ideal diode controllers may be a separate pin, but it will often be connected to either are wired for reverse protection and diode-ORing. These are generic the input or circuit supply. Some ideal diode controllers examples and do not represent specific devices, which vary, so provide status and/or fault condition outputs, which can be individual datasheets must be consulted for connection and operational read by a microcontroller. Some have control inputs, such details. The circuits in Fig.13 and Fig.14 use external N-channel as ‘enable’ to provide low-power shutdown mode. In load MOSFETs, but some sharing applications ideal diode controllers will regulate the ideal diode controllers MOSFET gate-source voltage to provide stable operation. M2 Fig.15 shows an LTspice schematic for an example circuit use P-channel devices, Supply2 and some have internal using an ideal diode controller. This uses the LTC4412 ‘Low MOSFETs. Compared Loss PowerPath Controller’ from Linear Technology (now In Gate Out to Fig.5, Fig.6 and from Analogue Devices) to implement switchover between a Ideal diode controller + Fig.8, the MOSFETs battery and a mains power adapter. A number of ideal diode Control Gnd Status in Fig.13 and Fig.14 controller ICs are available in the LTspice library. The circuit Circuit have their source and in Fig.15 is based on Fig.2 in the LTC4412 datasheet and is M1 drain reversed so that adapted from an LTspice file available from Analogue Devices. Supply1 – the body diode is in The simulation is similar to that in Fig.11 and Fig.12. The the same direction as mains adapter voltage (from V2) is ramped from 0 to 15V In Gate Out the P-channel device (then stays at 15V) while the battery voltage remains fixed at Ideal diode controller used above. This is not 14.4V throughout. When the sense pin voltage exceeds the a problem because the Vin pin by 20mV Q1 is switched off and the STAT pin goes Control Gnd Status MOSFETs can conduct low, turning on Q2 and allowing the mains adapter to power in either direction. the circuit. The results in Fig.16 show that the circuit voltage Ideal diode cont- remains almost constant, but switchover can be seen in the Fig.14. Ideal diode controller concept r o l l e r s h a v e p i n s current waveforms for the MOSFETs in the lower pane. circuit (diode-OR). Practical Electronics | July | 2023 55