PIE's zener + RCD clamp circuit schematic missing tie point or?

Hi bthurman123,

The voltage on cathode of TVS goes higher than Vdc input rail (the voltage on anode) because the reset diode is charging the reset capacitor Cc at the end of conduction time, for a short period of time (leakage discharge plus magnetic reset cycle). This amount of energy is not big but it is very important. To discharge the capacitor is that high value parallel resistor, to protect against very high spikes is the TVS.
There is more than one way to make scrambled eggs. Check the application notes and examples if you want to see different reset circuits.

Cheers,
PI_Crusher

PI, Even scrambled eggs need a current path, unfortunately there isn't one shown for TVS and RC in the PIE schematic attached.

So is this missing tie point bug?

Bill T.

Hi Bill,

The picture you posted is for the clamp circuit only. The left side is connected to the DC input rail in a real application, the bottom side is connected to the Drain of the switching element, and the right side is connected to the secondary output.
If you actually understood how this piece is connected in a normal schematic, I am afraid I have no idea what the question is and I can not offer any help.

Cheers,
PI_Crusher

Yes of course I am completely confused.

I thought the right side of the clamp was connected to the PRIMARY side of the transformer, not the SECONDARY side as you state.

However, still when I follow the the path from the cathode of the TVS to the anode of the TVS to the top of RC to bottum of RC back to the cathode of TVS.......I see no current path here.

Do you?

Bill t.

See my first answer; for now you just have to believe what I told you. You have to look a bit into this type of circuit, understand why you need a clamp circuit at all. It is absolutely normal to have this confusion and beside personal study there is no other way to overcome this barrier. Google around and see why a flyback transformer stores energy and how. This is the key to understand what is happening after the switch turns OFF. At that point it is a tiny amount of energy pumped as current trough the clamp diode (not the TVS) charging the clamp capacitor. The TVS starts conducting after a long period of time (dozens or hundreds of switching cycles), only after the voltage on the clamp capacitor builds up to step by step, all the way up to the zenner knee voltage. It is possible to fine tune the clamp resistor to subtract precisely the energy pumped by the clamp diode (lower the value of the discharge resistor RC) and in this case you can even remove the zenner (if the voltage on the clamp capacitor never increases to dangerous levels). It is desirable though to keep that zenner in order to provide increased reliability, even if the zenner never conducts. The closed loop is not local as you stated; in fact, that is not even a real path. The circuit closes trough the positive input DC voltage, GND, MOSFET, clamp diode, and clamp capacitor parallel with discharge resistor.

Cheers,
PI_Crusher

Hum, obviously I am not communicating well here.

The schematic that PIE generated here doesn't have the TVS or the RC connected to the positive bus.

When lines cross that are connected, a tie point shows that connection, otherwise they are not tied. (See the tie point at the bottum of CC, RC and TVS cathode.)

PIE has not generated a tie point between the TVS anode, CC and RC, therefore they are not connected at the top of the schematic.

This erronious schematic will lead someone astray eventually.

Bill T.