3L6565 – Block DiagramInput of the comparator, the outcoming signal depending on the comp and the VFF pinCurrent sense input, signal exceeds 2V will make the controller running into hiccup mode.Error Amplifier feedback input, FB regulate at 2.5VOutput Drive Capability 400mApkUVLO, Vcc turn-on 13.5V and turn-off 9.5VOutput of the current comparator block,It will be active when the current signal exceeds 2VZero Current DetectionOnce detect the negative going edge demag signal, it triggers the MOSFET turn-on.
4L6565 – Internal Supply Block Once the Vcc reaches the startup voltage, the controller begins to operate
5L6565 – Zero Current Detection Once the demag pin goes down below 1.6V, the EXT MOSFET will be switched on.To ensure high noise immunity, the triggering block must be armed first: prior to falling below 1.6V, the voltage on pin 5 must experience a positive-going edge exceeding 2.1 V.
7Major Parasitic Elements in Flyback Converter whereCd is the total capacitance of the drain nodeLlk is the leakage inductance of the transformerLm is the magnetizing inductance of the transformer
8VDS WaveformwhereThe first valley occurs at T = Tv, where Tv will be as the followingFor zero voltage condition, it implies that
9QR Timing and Power Relationship MOSFET ON time:MOSFET OFF time:Switching Frequency:Input Power:The switching frequency related to input power:where
10L6565 – Power Compensation, high line and low line
11Switching Frequency vs. Operating Condition In order to deliver the full loading power at minimum input voltage, the inductance should not exceed the following value:
12Switching Frequency Foldback The Tblank function is the function of the Vcomp output
13L6565 – Syn-Rect Demoboard Circuit QR mode is the perfect topology for syn-rect because secondary currentflow stops before primary MOSFET switches on!
14Low Startup CircuitThis part is for startup purpose use. The power loss dissipation is much reduced compared with the traditional lossy R-C startup circuit.The charge current pump into the Vcc capacitor at a rate of
15Isolation Gate Drive Transformer L202 – isolated gate drive transformerThe gate drive voltage is converted from the secondary current.When the secondary current is large enough, it turns on the Q203, then an output voltage is directly going into the totem pole input and drive the secondary MOSFET.When the secondary current drops to zero, there will be no driving voltage from the isolated gate drive transformer, and Q203 turn-off and the MOSFET will not turn on.Secondary currentIsolated gate drive transformerMake use of syn-rect approach, efficiency improves significantly!
16Operating Waveforms Vin = 240Vac, Vo=12.3V and Io=5A Primary currentVdrainVin = 100Vac, Vo=12.3V and Io=5APrimary currentVdrainVin = 240Vac, Vo=12.3V and Io=5A
17Operating Waveforms Vin = 100Vac, Vo=12.3V and Io=5A Primary currentPrimary currentVdrainVdrainVin = 100Vac, Vo=12.3V and Io=5AVin = 240Vac, Output Short Circuit