@Glen...well I got most of that right - centre tap transformer, non-overlapping FET drive, alarm on either output not going high enough, etc. Glad I am not tooo stupid yet. But you have not told us exactly what this circuit is for? What is on the secondary side of the transformer?
And thanks for that - I love stuff like this. Any more of them will be very welcome.
@David What is on the secondary side of the transformer?
Don't know. All I know is one of our sales people promised a customer that we could repair absolutely anything and the boards showed up - no chassis, no backplane, no transformer, no information other than they were from some piece of legacy telecom power supply. We made a test fixture once we understood what these boards are supposed to do.
Was tempted to jump into the discussion but didn't want to spill any cool beans too soon. It was great to see how everyone went through similar thought processes based on the little bits of information gleaned from the schematic.
Glad you like puzzles like this, have another one in the works. But as Max said, anyone with puzzles of their own can send them to him.
Typically in a push-pull converter the secondary is rectified and filtered to produce a DC voltage. What is neat about this circuit is that the output voltage is sensed on the primary side by the filtered peak detectors and used to assert the overload ALARM signal when the output voltage drops a certain level caused by a heavy load.
I imagine one could next use the ALARM signal to trigger a pulse generator that resets the flip flops for some time creating some cooling off period (i.e. hiccup mode) or just latch the signal requiring a system reset or power cycle to recover.
My guess is that this part of a system for providing back-up power to the instrumentation on an air-craft from a 48V battery, where it is essential to know WHEN the battery is being drained. This schematic is the power transformer driver.
What is not shown on the schematic is the transformer. EdgeConn1 and EdgeConn2 are permanently connected to the ends of the primary windings of a centre-tapped transformer, and the centre-tap is connected to the 48V battery via a switch. The switch closes (perhaps automatically) if the primary source of power fails.
Normally the circuit can be under continuous operation (so that is can be checked it is working), but the alarm signal only becomes asserted with the load is connected (the primary CT is connected to the battery).
Load Handling Capability: Current: The main MOSFETs are quite hefty (TO-220, 200V, 0.18ohm, 11A continuous drain current at case temp of 100C). They are also quite heavily protected from over voltage (CR8, 110V 5W) and there is a heavy dv/dt snubber (C3 and R5) which is reset by R6 (R6 and C3 has time constant of about 1.4ms, which is about the OFF time of each MOSFET). So I suspect this circuit is intended to drive a substantial load which can be inductive. I'll make the assumption that the MOSFETs are fitted to heatsink, and assume the heatsink is capable of dissipating at least 10W total while limiting temperature rise to about 50C. Assuming MOSFET case temp is at 100C when operating at full power, this causes Rdson to increase by about 1.5, so Rdson=1.5 x 0.18 = 0.27ohms. For 10W total power dissipation, I = Sqrt (10 / 0.27) = 6A max load current per MOSFET at 50% duty.
Voltage: Each MOSFET drain node can only handle 100V max, limited by the clamping zeners. So the loads can be up to 100V if resistive, but if inductive then they are limited to half of this voltage to allow the volt-seconds to balance. The anti-phase drive and the low power rating of clamping zeners suggest that Single-ended inductive loads are not likely; more likely is coupled inductive loads, such as a transformer with a centre-tapped primary, in which case the centre-tap voltage can be up to 55Vdc; note that this node is not on this circuit.
So total power rating is 55V x 6A, let's call it about 300VA.
Both frequency and duty-cycle are fixed (there seems to be no form of feedback to modulate either of these) so there is no intention to regulate the final output at the load.
Many of these showed up one day, I think the customer was 'bonepiling' them until running low on spares, or simply could not find a place to repair them. Now that we know what they are supposed to do and have a test fixture we can continue to provide this service.
You're right, this was easy in comparison to to some of the unknown boards that sales promises we can handle. ("Can we fix this? I hope so because I just told the customer to send in 20 of them!")
Glad you enjoyed the puzzle. Have a couple more in the works, but as Max says, anyone with a puzzle idea can send it in to him.