P7 - Power Supply Built
I simplified the schematic in the previous post and built up the circuit on a breadboard.
Here is Schematic A.
I have also included the basic design information in the schematic.
UPDATE: I have redone the schematic again. I now bypass the error amplifier in the UC3845. I removed the +15VDC and +20VDC secondary to simplify testing. I have included snubbers on the primary side.
Here is Schematic C.
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The Board. VAC comes in at the top left. The top row is the line filter and AC rectification. The second row has the controller, the power switch and the transformer output. The third row has the optoisolator and feedback control logic.
I wound the transformer by hand. It has five separate windings (primary, bias, +5V, +15V, +20V). Each winding is insulated from the other (my first attempt was prettier but it shorted out during a current spike -- always insulate!). The transformer is 'gapped' with 4 pieces of electrical tape. The exact gap was a process of trial and error. I used an RLC circuit to measure the inductance. I had computed that I needed a 450uH inductor so I simply kept adding more gap (more tape) until I got the inductance to come down to 450uH. Of course, this added to the leakage inductance (~26uH). |
 Built PSU Schematic A - No Snubbers |
 Built PSU Schematic C - With Snubbers |
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Here is a picture of the MOSFET drain-to-source voltage (Vds) while regulating a 500mA load on the +5V line. The yellow trace is my Vds and the blue trace is the MOSFET gate voltage.
If you know anything about these waveforms then you know this one is a little bit of a mess. What is happening is that during the on-time (when the blue trace is low), the current in the inductor is building and the current in the output (the secondary) is blocked by the secondary diode (the output capacitor holds the voltage). When the MOSFET turns off, the voltage on the primary reverses/spikes to nearly 600V!. This starts the secondary current which build for about 1uSec, then decays for about 4.5uSec. When that ends, the controller goes into discontiguous mode (runs out of current) and it decays to the input voltage over the remainder of the cycle. To fix this, I need to adjust the controller so that is uses more of the switch period (closer to contiguous mode) and add a primary snubber circuit to clip off the 600V spike. |
 Yellow = Vds Voltage Blue = Gate Schematic A - No Snubbers |
 Yellow = Vds Voltage Blue = Gate Schematic C - With Snubbers |
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Here is a picture of the MOSFET source current as measured through the sense resistor. The sense resistor is 0.25 Ohms so dividing any voltage reading by 0.25 will yeild the current.
The current ramps from near zero at -6.72uSec (-6.72uSec, -16mV) up to 576mV at -0.28uSec (-0.28uSec, 576mV). During this time, the voltage across the inductor is 150V (nearly. There is a drop across the switch because this switch has a fairly high Rds ~= 2.2Ohms). The voltage across the inductor is equat to the inductance times the change in the current divided by the time (v = L * di/dt). Using this equation, it follows that the inductance is 431uH (=150 * 6.4uSec / 2.24A). This is close to the inductance that was measured using an RLC circuit (=467uH) (at least, within the measurement capabilities of my equipment). Notice the ugly spikes of current at the start and the end of the ramp. They go well over 4Amps! These have to be cleaned up. This inject a lot of noise into the output. |
 Yellow = Ids Blue = Gate Schematic A - No Snubbers |
 Yellow = Ids Blue = Gate Schematic C - With Snubbers |
| +5VDC Output. Both of these pictures were taken with Schematic C (snubbers included). The left picture does not include the output inductor (L2 - 2.2uH). The right picture does. |
 Yellow = Vds Blue = +5VDC Output Schematic C - With Snubbers - Without Output Inductor |
 Yellow = Vds Blue = +5VDC Output Schematic C - With Snubbers - With Output Inductor |
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