L6565 – Block Diagram Zero Current Detection Once detect the negative going edge demag signal, it triggers the MOSFET turn-on. Output Drive Capability 400mApk Error Amplifier feedback input, FB regulate at 2.5V Current sense input, signal exceeds 2V will make the controller running into hiccup mode. Input of the comparator, the outcoming signal depending on the comp and the VFF pin UVLO, Vcc turn-on 13.5V and turn-off 9.5V Output of the current comparator block, It will be active when the current signal exceeds 2V
L6565 – Internal Supply Block Once the Vcc reaches the startup voltage, the controller begins to operate
L6565 – 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.
Zero Current Detect Operation
Major Parasitic Elements in Flyback Converter where Cd is the total capacitance of the drain node Llk is the leakage inductance of the transformer Lm is the magnetizing inductance of the transformer
V DS Waveform The first valley occurs at T = Tv, where Tv will be as the following For zero voltage condition, it implies that where
QR Timing and Power Relationship MOSFET ON time: MOSFET OFF time: Switching Frequency: Input Power: The switching frequency related to input power: where
L6565 – Power Compensation, high line and low line
Switching Frequency vs. Operating Condition In order to deliver the full loading power at minimum input voltage, the inductance should not exceed the following value:
Switching Frequency Foldback The Tblank function is the function of the Vcomp output
L6565 – Syn-Rect Demoboard Circuit QR mode is the perfect topology for syn-rect because secondary current flow stops before primary MOSFET switches on!
Low Startup Circuit This 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.
Isolation Gate Drive Transformer L202 – isolated gate drive transformer The 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. Isolated gate drive transformer Secondary current Make use of syn-rect approach, efficiency improves significantly!
Operating Waveforms Primary current Vdrain Vin = 100Vac, Vo=12.3V and Io=5A Primary current Vdrain Vin = 240Vac, Vo=12.3V and Io=5A
Operating Waveforms Vin = 240Vac, Output Short Circuit Primary current Vdrain Vin = 100Vac, Vo=12.3V and Io=5A Primary current Vdrain
Transformer Design (I)
Transformer Design (II) In Quasi-Resonant mode control, the current always starting from zero of every switching cycle.