Solution | Twelve articles: Anti-counterfeiting, which method is more suitable for you? Learn about ideal diode controllers

Latest update time：2023-12-11

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In power systems, it is usually necessary to design reverse polarity protection to prevent on-site power wiring errors and avoid burning circuit board components. Most electronic components do not support reverse connection of the positive and negative poles of the power supply, and Schottky diodes are a simple and cost-effective circuit anti-reverse solution.

It utilizes the unidirectional conductivity of the diode and does not require additional circuitry to directly pass it into the positive electrode of the power supply, thus preventing reverse conduction and increasing power capacity through multi-channel power supply. However, due to the conduction characteristics of diodes, using them as anti-reverse circuits will inevitably lead to problems of large conduction voltage drop and obvious power loss under high current , which requires the addition of additional thermal management systems for heat dissipation.

Comparison of three anti-reverse circuit designs

In the design of front-end power systems, modules or subsystems that are directly powered by batteries need to be designed to protect against reverse battery connection or dynamic reverse polarity conditions when inductive loads are disconnected from the battery. Especially in the automotive battery power supply scenario, the battery power supply environment caused by the complex body system is more complicated. In addition, post-maintenance and battery reinstallation scenarios have a higher frequency. Once the connection is reversed, the connected subsystems, circuits and Components were damaged, causing significant losses.

During the use of the vehicle, combined with the electrical transient specifications indicated by ISO 7637-2 or ISO 16750-2, the vehicle power supply interference situation can be seen more clearly, as shown in the figure.

Figure 1 Car battery system power supply interference

Use diode anti-reverse

Option One

Taking the passenger car 12V battery system as an example, when the 12V input quickly reverses to -12V, the output voltage remains unchanged and will not cut off immediately because the Schottky diode will be reverse biased and cause the output to be incompatible with the negative voltage. separated. The bulk capacitor at the output will prevent the output from dropping immediately and power the load for a short time before input power is restored .

Figure 2 Schottky diode single-phase conduction anti-reverse circuit

This solution has the following shortcomings due to the diode characteristics:

1. Power consumption problem : At higher load current, forward conduction will cause significant power consumption;

2. Heat dissipation problem : A radiator or additional PCB heat dissipation design is required to manage the heat generated by the diode power consumption, thus increasing cost and space;

3. System voltage drop problem : In system design, when the input voltage is low, it is necessary to maintain the stability of the output as much as possible, especially when doing voltage drop experiments, and the conduction voltage drop problem of the diode causes the actual system power supply chip of DC/DC to The input voltage becomes smaller.

Use PMOS anti-reverse

Scenario 2

In order to reduce the forward voltage drop of the diode, the Schottky diode can be changed to a PMOS solution. The typical circuit is as follows:

Figure 3 PMOS anti-reverse circuit

Replace the diode with P-channel MOSFET Q1 and make its body diode D1 conduct in the same direction as forward. During normal battery operation, the body diode D1 of MOSFET Q1 will be forward biased and conduct for a short time, after which the gate voltage is pulled below the source to the Vgs(th) turn-on voltage which will turn MOSFET Q1 on. . When the battery polarity is reversed, the gate-source voltage becomes positive and turns MOSFET Q1 off , protecting downstream circuitry from negative voltages.

Of course, the PMOS anti-reverse circuit solves most of the anti-reverse defects of Schottky diodes, but it still has the following shortcomings:

1. Cost and selection: For high-voltage and high-current PMOS, the cost is higher and the optional parameter specifications are fewer than that of NMOS;

2. Standby power consumption : The Vgs drive and protection circuit composed of a voltage regulator tube and a current-limiting resistor all have leakage current , so the selection of the current-limiting resistor R2 will affect the overall standby power consumption.

3. Backfeed current : When the input power drops, the PMOS channel is still conductive. At this time, the voltage of the system capacitor will feed back into the power supply, causing the product's system to lose power and interrupt its function; when testing with ISO 7637-2 input superimposed AC voltage , since the PMOS is fully turned on, there is also the phenomenon of current backflow, and the RMS current of the output electrolytic capacitor will generate additional heat on the capacitor due to ESR.

Use NMOS anti-reverse

third solution

The gate drive circuit of connecting NMOS in series on the low side is simple , and the cost of NMOS is low . The typical circuit is as follows:

Figure 4 NMOS low-side anti-reverse circuit

Since not all systems can withstand system ground voltage jumps during on/off or load current transients, and this anti-reverse structure causes the power ground and load ground to be separated, the signal integrity will be greatly affected, and the circuit's EMC and EMI characteristics also change , so they are rarely used in automotive electronic product design.

However, if NMOS is used to prevent reverse connection at the positive terminal of the power supply, it can not only prevent the input power from being reversed, but also block the reverse current from flowing back to the input terminal from the output load. This ideal diode controller + NMOS is obviously a better solution .

What is an ideal diode controller?

Ideal diode controllers provide key features such as low operating quiescent current, ultra-low shutdown current, regulated forward voltage, and fast reverse current response , allowing them to emulate ideal diodes in a variety of applications. The power MOSFET is connected to ensure that its body diode blocks reverse current when the MOSFET is turned off. Since the MOSFET conducts during forward conduction, the forward voltage drop and power dissipation will be significantly reduced . An ideal diode controller detects reverse current through the MOSFET and turns the MOSFET off , allowing the body diode to block the reverse current .

Product analysis

Xinzhou Technology SCT53600Q is a 65V ideal diode controller with an internal charge pump that can fully drive the MOSFET gate level higher than the anode during normal operation and turn on the forward comparator when reverse current is detected. And turn off the reverse current comparator so that the MOSFET body diode completely blocks the reverse DC current. The typical application circuit is as shown in Figure 5:

Figure 5 12V battery system positive anti-reverse circuit

Forward conduction loss

Compared with Schottky diodes, with the same load current of 10A, the power consumption of the diode is about 5W, while the conduction loss of a 5mΩ Rds(on) NMOS is only 0.5W, which can save about 10 times of energy .

Fast load response

When the forward voltage drop is greater than 50mV, the controller has a peak source current of 7mA, which can quickly charge the gate during fast load transients , thereby minimizing the power supply voltage drop , as shown in the figure. The load quickly changes from 0.1mA to light load. Load transient response of SCT53600Q when switching to 3A:

Input micro short circuit

When an input micro-short circuit occurs at the input, the SCT53600Q will respond quickly within 1us to turn off the MOSFET, thereby preventing reverse current from flowing back to the short-circuited power supply. As shown in the figure, during the input short circuit, the output will remain on, and the input capacitor supplies power to the load until the micro short circuit is restored .

static reverse polarity

When the input power is connected in the reverse direction, the SCT53600Q is in a shutdown state to prevent downstream circuits and components from being damaged . The output voltage is still protected. The shutdown waveform is as shown in the figure:

dynamic reverse polarity

Based on the test waveform of ISO 7637-2 Pulse1, it simulates the situation where the voltage at both ends of the inductive load will be reversely applied to the power supply end of when the power supply is suddenly disconnected, which can effectively prevent reverse reaction. The waveform is as shown in the figure:

For power systems that need to prevent input reverse connection, the design solution of ideal diode controller + NMOS can achieve better energy-saving design, reverse current blocking, and increase voltage input margin. Especially in automotive systems, it can better pass ISO 7637-2 test.

SCT53600Q