Design Ideas January 16, 1997

Solid-state flasher has short-circuit protection

Navdhish Gupta, TDF Corp, Naperville, IL

The circuit in Figure 1 switches an inductive or resistive grounded load using a bootstrapped, n-channel, power MOSFET. The circuit features short-circuit, reverse-polarity, and transient-input-voltage-spike protection. The input voltage, V_DD, can vary from about 18 to 30V, and the circuit can handle as much as approximately 10A with proper heat sinking of the FET. The circuit flashes at a rate of about 100 flashes/minute, with a duty cycle of about 47%. You can use this circuit as a solid-state flasher for vehicles with 24V nominal-power systems.

The circuit operates by setting up the CMOS 555 timer as a free-running, monostable oscillator with a duty cycle of roughly 50% and a frequency approximately equal to 1/(1.53R₁3C₁). This oscillator drives Q₁, which in turn drives Q₄. When Q₁ is on, Q₄ is off, and when Q₁ is off, Q₄ turns on, setting the load high. Because Q₄ is an n-channel device and performs as a high-side switch, for good turnon of the FET (that is, low R_DS@-(ON)), V_GS(ON) should be greater than 10V. To accomplish a higher gate voltage than the supply voltage, D₁ and C₂ bootstrap the voltage at the gate of Q₄. D₃ protects the gate to source of Q₄ from voltages as high as V_DD that the bootstrapping produces. Note that using a p-channel power FET would be easier for switching a grounded load, but would cost quite a bit more.

Under normal operating loads, Q₂ and Q₃ are both off because the cathode of D₄, which ties to the base of Q₂ through D₂, is always high. (D₂ increases the effective reverse-breakdown voltage of Q₂.) The cathode of D₄ alternates between approximately V_P and V_LOAD, which keeps Q₂ off and, therefore, Q₃ off as well. If the load is short-circuited, however, the voltage at the cathode of D₄ alternates between V_P and ground, which causes Q₂ and, consequently, Q₃ to turn on whenever Q₁ is off and the FET is trying to turn on. Q₃ brings the gate voltage low enough to effectively turn off the FET. Note that when Q₁ is on, the FET is also off. R₂ and C₃ provide a small delay before Q₃ turns on, which enables the circuit to start up during normal operation.

R₃ and R₄ adjust the sensitivity of the short-circuit-protection circuitry. A very fast-acting protection scheme will be at the expense of circuit start-up at lower V_DD, voltages, and, especially, at temperatures higher than 1408F because of the decrease in the base-emitter diode voltage with increasing temperature. R₅ provides some feedback to counter this effect. R₆ senses shorts that are at a distance, that is, shorts that may be caused by long wires with some significant resistance (but obviously smaller than the load).

Transient suppressor TS₁ protects the circuit from transient spikes on the input power. The internal diode in the FET safely channels as much as 10A of reverse current if the polarity is reversed on the inputs. To cut EMI emissions, you can add C₄. C₄ and R₇ smooth the corners of the output square pulses at the expense of higher power dissipation in the FET. You can wrap the outputs around a ferrite core (configured as a balanced choke) to further reduce conducted emissions. (DI #1980)

Figure 1

This solid-state flasher switches an inductive or resistive grounded load using a bootstrapped n-channel power MOSFET and features short-circuit, reverse-polarity, and transient-input-voltage-spike protection.

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