ST AN1215 Application note

AN1215

APPLICATION NOTE

HOW TO HANDLE SHORT CIRCUIT CONDITIONS

WITH ST’s ADVANCED PWM CONTROLLERS

by Claudio Adragna & Giuseppe Gattavari

Purpose of this note is to provide some advice on how to manage a short circuit condition for a switching converter controlled by ST's advanced PWM controllers L5991 and L5993.

After a quick review of merits and limits of the current limitation functionality embedded in these IC's, a simple circuit will be given that allows to fulfill designer's typical requirements on what the converter is supposed to do in case of overload or short circuit.

Current limitation schemes: merits and limits

ST's advanced PWM controllers L5991 and L5993 [1] [2] include a two-level overcurrent protection: the "pulse- by-pulse" (or "cycle-by-cycle") current limitation and the so-called "hiccup mode" operation.

Pulse-by-pulse current limitation ideally limits the peak primary current at 1/Rs (see figure 1) when the converter is overloaded or there is a load short circuit. Even neglecting non-idealities, which worsen the scenario and must be accounted for in a safe design, pulse-by-pulse current limitation just prevents the system from losing control of the peak primary current. Nothing more.

Figure 1. PWM control and current limiting in L5991 and L5993.

				Vac	Vin
					Is
					Vout
					Lp
2.5 V	ERROR		CLOCK
AMPLIFIER			PWM		Vdrain
		COMPARATOR
+	2R			S
			-
-		1V	R	Q	DRIVER
	R		+		Ip
				LATCH
	ADVANCED PWM		+		Vcs
	CONTROLLER				Rs
				HICCUP
	(L5991, L5993)		-
					ISOLATED
		1.2V	SOFT-START		FEEDBACK
				Css
December 1999					1/7

AN1215 APPLICATION NOTE

The output current is not limited and may cause PCB traces burning. Converter's power throughput declines only slightly. Furthermore, power losses get concentrated on some specific parts, e.g. the output rectifier(s), which may overheat and go towards destruction if not rated for this extreme condition and if heatsinking capability is not generously provided.

When pulse-by-pulse current limitation cannot maintain the peak primary current under control, hiccup mode protection takes over and halts the resulting current runaway. This happens when the switch ON-time needed to control the peak primary current is lower than the minimum value TONmin that the controller can generate as a result of its internal delays. In L5991 and L5993 TONmin can be estimated as high as 200 ns, typical value. More details on this "runaway condition" can be found in the appendix.

With reference again to figure 1, in case of current runaway the voltage on the current sense pin will go over 1V. As it reaches 1.2V a comparator triggers this special functionality [1], [2].

Unlike pulse-by-pulse limitation, hiccup mode operation keeps output current and power throughput very low. It would be desirable, then, to rely on hiccup operation to conveniently handle a load short circuit. However, it is quite common that even a dead short at the converter's output cannot activate hiccup protection.

Referring to the appendix for a detailed analysis, high input voltage, high regulated output voltage and high switching frequency favour current runaway and, therefore, hiccup operation to be tripped further to a load short circuit. In many applications, such as AC-DC adapters or PC's silver boxes, where the output voltage is not so high and the switching frequency is quite low (below 100 kHz) it is extremely likely that the runaway condition will never be met and the hiccup protection will never be activated. Furthermore, in these applications the load is usually connected through long cables, then the total short circuit resistance is not low and the TON needed to control peak current needs not be so short.

In other applications, such as monitor SMPS, the runaway condition is likely to be met for the highest output voltages, especially at high input voltage and if they are synchronized at high frequency.

Actually, hiccup protection serves two different purposes. The first one is related to the capacity of the L5991 and L5993 to be operated at a very high frequency (up to and beyond 500 kHz): to guarantee a safe handling of a load short circuit under all conditions of operating frequency, hiccup protection must be added to pulse-by- pulse limitation. The second purpose is to cover some fault conditions that pulse-by-pulse limitation cannot handle. These are basically:

1.short circuit of the secondary winding (regardless of the topology);

2.secondary catch diode short circuit (in flyback converters);

3.secondary freewheeling diode short circuit (in forward converters).

Figure 2. Fault conditions that activate hiccup mode operation.

Vin	Vin	Vin	Vin
LLK	LLK	LLK	LLK
LP	LP	LP
LM	LM	LM

Secondary winding	Secondary diode	Freewheeling diode	Equivalent circuit
short circuit	short circuit	short circuit
(any topology)	(flyback topology)	(forward topology)

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AN1215 APPLICATION NOTE

All of these conditions reflect a short circuit to the transformer's primary side while the switch is ON. The primary current rate of rise is then limited only by the leakage inductance of the transformer, which is few percent of the primary magnetizing inductance (see fig. 4). This means that the current slope will be 30-50 times higher than in normal operation, thus pulse-by-pulse limitation has no chance to control the current.

Overcoming pulse-by-pulse issues

With the aim of reducing power consumption during an overload or a load short circuit, it is possible to select one of the following two basic strategies:

1.Making the converter enter hiccup mode operation as a fault is detected and letting it go back into normal operation automatically as the fault is removed (autorestart).

2.Shutting down the converter as a fault is detected and keeping it off until it is disconnected from the mains. Only after a power-off / power-on cycle the converter is enabled to restart.

Both of these strategies can be easily implemented with ST's advanced family of PWM controllers, thanks to the numerous functions available. Figure 3 shows how this can be done with the same basic circuit comprising only four cheap external parts.

Figure 3. Different strategies for low dissipation during load short circuit

	BC327	3.6 kΩ				BC327	27 kΩ
Vref			10	µF	Vref				0.47 µF
6	4		1N4148		6	4		1N4148
	L5991					L5991
COMP					COMP
	L5993	ISEN	1.5 kΩ			L5993	DIS	R6 330 kΩ
5	12	13			5	12	14
VFB	SGND			Rs	VFB	SGND		R5
								47 kΩ	OVP
	Autorestart					Shutdown

When in current limitation because of an overload or a load short circuit, the output voltage drops below its regulated value and the output of the error amplifier (pin 6, COMP) saturates high (³ 6V) in the attempt of recovering voltage regulation. This is used for turning on the small-signal PNP (its base is tied at 5V) and getting a current available for triggering a protection functionality. However, a delay of several ms is needed to prevent the function from being activated at start-up or as a result of a load surge. This is simply done with an RC cell. Finally a diode is needed to decouple the circuit from the others connected to the same pin, during normal operation.

Hiccup is activated by pulling the current sense pin (ISEN, 13) above 1.2V. Error amplifier source capability with high saturation voltage is about 1 mA, thus the resistor between the current sense resistor and ISEN pin is increased from 1 to 1.5 kW. The 3.6 kW collector resistor and the 10µF capacitor form a delay cell of about 15 ms.

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