Understand how to design and configure a latch which has this use, and many others
(Editor's note: to see a linked list of entries #1 to #37 in the Power Tips series, click here; to see a linked list of all entries, click here.)
Have you ever needed a simple, inexpensive latch circuit? Figure 1 shows one that can provide fault protection in power supplies with only pennies of components. It is basically a silicon controlled rectifier (SCR) implemented with discrete components.
Figure 1: A SCR with controlled holding current is built with discrete components.
The two transistors normally are off. To fire the latch, you drive either the base of the PNP low or the base of the NPN high until one of the transistors turns on. This creates collector current flow and turns the other transistor on, which further turns the original transistor on.
The circuit latches in a regenerative way. Current is only limited by the source impedance and the transistor characteristics, allowing this circuit to rapidly discharge a capacitor.
An interesting characteristic of this circuit is that you can establish the holding current of the SCR by choosing the resistor values. For the latch to remain on after being fired, there has to be sufficient voltage (~0.7 V) across the two base-emitter junctions to keep them on. This means that the circuit latches, if it is supplied with a current of at least Vbe / R1 + Vbe / R2.
If the latch is connected across a capacitor that is fed with a small current, the latch discharges the capacitor. Once the current in the circuit falls below the holding current, it turns off.
Figure 2 shows one place where this circuit can be put to good use. This is a high-voltage input, 48-volt output flyback converter that uses the SCR to shut down the power supply in the event of an output overvoltage condition caused by a fault in the control circuitry.
Figure 2: The SCR can be programmed to latch or not.
When an input voltage is first applied to the circuit, current through R3 and R4 charges the bulk capacitor C3. When the voltage builds sufficiently high on C3, the control IC begins operation by switching power FET Q3 and transferring energy to the output.
The output voltage is regulated by controlling current in U1, which controls the energy transferred through the transformer. This circuit provides isolated overvoltage protection through U3. Zener diodes D5 and D6 are chosen so that they do not conduct during normal operation. In the event of an overvoltage, they conduct, forcing current through the optocoupler, U3 which triggers the latch, composed of Q4 and Q5. The latch discharges bias capacitor C3 and U2 ceases to operate when the VDD voltage reaches U2’s under-voltage lockout point.
The latch continues to discharge the bias capacitor until the voltage reaches near 1 V. It is at that point that the values of R3, R4, R14 and R16 become important. R3 and R4 limit the current available from the input line and R14 and R16 determine how much holding current is required in the latch. If R14 and R16 are small in value, the latch turns off and the bias capacitor recharges, and the power supply tries to provide output power again.
This choice provides a continuous retry in the event of a fault. If the resistors are sufficiently large in value, the latch remains on and the power needs to be cycled to reset it. In this case, there is not a continuous retry.
Another important component in this circuit is R5, which limits the bias supply power after the latch has fired. Normally, this part is needed to prevent peak detecting of the bias voltage.
This circuit can be used in a number of ways, particularly since you can use a rising or falling edge to trigger it. For instance, the overvoltage protection can be implemented on the primary side by connecting a Zener diode between the bias voltage and the base of Q5. You can use a temperature sensor with a negative going transition to drive the base of Q4. Or you can use a comparator on the secondary side to provide a very accurate over-current shutdown with an optocoupler very similar to Figure 2.
To summarize, this latch composed of $0.03 of transistors is very versatile. It can be triggered by either negative or positive transitions, and it can be latching or non-latching, depending on your resistor value choices.
Please join us next month when we will compare transient response of discontinuous and continuous power supplies and show that efficiency is not the only reason to use a synchronous rectifier.
For more information about this and other power solutions, visit: www./power-ca.
About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.
