高方面驱动器增强了短路保护|电子设计

This circuit builds on a previous idea in which the addition of an optocoupler to a microcontroller output converted a “dumb” high-side driver into a “smart” one with diagnostics and short-circuit protection.¹

The circuit adapts this concept for cases where a microcontroller is either not required or inappropriate. It also adds short-circuit load protection to an even “dumber” driver such as the output of a simple logic gate(Fig. 1). As a result, it too allows a “dumb” low-level device to control a high-power load operating at a much higher voltage without fear of destructive short-circuit conditions.

1. The simple high-side driver can protect even a basic logic gate against load short circuits by controlling the optocoupler turn-on and turn-off periods.

假设C1最初是未充电的，驱动程序输出为0 V，而N通道MOSFET Q1关闭。当驱动程序输出从低水平转变为高水平时，脉冲通过C1和R2偶联到Q1的门，将其打开。如果没有通过OptoCoupler U1提供的反馈，电容器C1将通过R1迅速充电，Q1的栅极电压将下降到零，并且MOSFET将在大约一毫秒后关闭。但是，Optocoupler的存在有效地使电路可以监视整个负载的电压。

如果负载是正常的,完整的电源电压(V +_S) appears across it when Q1 turns on, forward-biasing U1’s photodiode via R3. This turns on U1’s phototransistor, which shunts C1, pulling Q1’s gate up to the high-level drive voltage. Since C1 is now effectively “clamped” by U1’s phototransistor, it cannot charge up, so Q1 remains on and the load remains energized.

If the load is short-circuited at any time by a fault, though, U1’s phototransistor turns off, C1 rapidly charges via R1, and Q1’s gate voltage quickly falls to zero. The MOSFET now turns off, disrupting current flow to the faulty load, and remains off until the driver output is cycled low (to allow C1 to discharge) and then high again. The circuit will continue to “reset” to the off condition until the load fault is removed.

Resistor R2 provides current limiting to prevent Q1’s gate-source capacitance damaging the driver’s output at turn-on. It may also be necessary to prevent the MOSFET from oscillating. A value of a few hundred ohms is usually suitable.

Figure 2 shows the operation of a circuit built with a BUK455-60A MOSFET for Q1. Load-supply voltage +V_S为12 V，负载为100Ω，驱动信号为60 Hz，5V方波。

In normal operation(Fig. 2a)，驱动信号处的高水平在Q1的排水端子处导致低水平（整个负载越过全电源电压下降）。在图2B中，负载已短路。

2. Normal operation has a low level at Q1âs drain terminal and the full supply voltage across the load (lower trace), due to a high level at the drive signal (upper trace) (a).When the load is shorted, the full voltage is impressed across the load for under a millisecond (b). Driver voltage is 2 V/div, Q1 drain is 5V/div, and the horizontal axis is 2 ms/div.

When the driver signal goes high, the circuit output at Q1’s drain terminal falls to zero only for a very brief time (less than a millisecond) and then immediately rises back up to +12 V. In other words, the full supply voltage only appears across the faulty load for less than a millisecond until the circuit “resets,” causing the voltage across the load to drop to zero.

在负载故障的情况下，C1/R1时间常数在很大程度上决定了简介“ ON”脉冲的持续时间。如果此时间常数太短，则在正常负载的情况下，OptoCOUPLER将没有时间正确打开夹紧C1。但是，如果时间常数太长，则短路负载可能会导致在简介“ ON”脉冲期间超过Q1的脉冲电流，并可能损坏MOSFET。

Using C1 of 22 nF and R1 of 56 kΩ worked well in the test circuit. An optional current-limiting resistor R_CL(usually a few ohms) in series with Q1’s drain may be advisable to limit the maximum output current to a safe level.

Reference

1. “Add Short-Circuit Protection, Diagnostics To Automotive High-Side/Low-Side Driver,” Vishwas Vaidya,电子设计, March 7, 2013.

安东尼·史密斯has been working as a consultant engineer for the past decade, designing products for the industrial, domestic, and automotive markets. He received a BSc (Honors) in electronics from Salford University, Greater Manchester. He also holds two patents. He can be reached at tony.scitek@tiscali.co.uk.