An alternative to current-transformer-based designs is to use a pair of comparators that differentially sense the MOSFET's drain-to-source voltage to determine the polarity and level of the current. This dc-coupled measurement can drive logic that turns the power device on and off near the zero-current transitions. No PLLs or external timing sources are necessary, and the control circuit operates on a unipolar supply (Fig. 2b).
Using the power MOSFET's on-resistance as a measurement shunt, the input comparators sense the rectifier current. Internal blanking logic prevents spurious transitions and guarantees proper operation in continuous-conduction mode (CCM), discontinuous-conduction mode (DCM), and critical conduction mode (CrCM) (see "Modes of Conduction"). This flexibility means that the secondary-circuit control method doesn?t constrain your choice of primary-circuit operation mode.
Modes of operation for a flyback circuit differ mainly for the turn-off phase of the SR switch. On the other hand, the turn-on phase of the secondary switch, which corresponds to the turn-off phase of the primary-side switch, is identical. This makes possible a variety of converter control schemes, including fixed-frequency, quasiresonant; variable-frequency; and fully resonant, actively clamped converters with switching frequencies as high as 500 kHz.
Turn-On Phase At the start of the SR FET's conduction phase, current begins to flow through its body diode, generating a negative drain-to-source voltage across it. The body diode maintains a higher voltage drop than that of the device's drain-source channel. Therefore, it triggers turn-on threshold voltage VTH2 (Fig. 3).
At that point, the control logic drives the MOSFET's gate on, which in turn causes the conduction voltage (VDS) to drop. Some ringing usually accompanies that voltage drop, and the ringing can trigger the input comparator to turn off. This can be dealt with by using an externally programmable minimum on-time (MOT) blanking period that maintains the power MOSFET in the on state for a minimum interval. The programmable MOT also limits the SR MOSFET's minimum duty cycle and, as a consequence, the maximum duty cycle of the primary-side switch.
The synchronous MOSFET's turns-on and -off behavior closely emulates the diode's function, thanks to the use of the same device as the sensing element. This approach obtains the highest possible performance for a given switch, often enabling the use of smaller switches. The control resolution of a discrete implementation is often insufficient to measure the current waveform close to its zero crossing, allowing the current to invert before switching off (Fig. 4a).
DCM/CrCM Turn-Off Phase Once the SR MOSFET turns on, it remains on until the rectified current decays to the level at which the drain-to-source voltage ( VDS) crosses the turn-off threshold voltage VTH1. How this action takes place depends on the mode of operation.
In DCM, the current crosses the threshold with a low di/dt. Once the current crosses the threshold, it once again flows through the body diode, causing a negative step in VDS . Depending on the amount of residual current, VDS might again trigger the turn-on threshold. To prevent this, an internally set blanking interval (tBLANK) causes the controller to ignore this VTH2 crossing. As soon as VDS crosses positive threshold VTH3, this blanking time terminates, and the controller is ready for the next conduction cycle.
Control method comparison Although implementing this scheme using discrete components would be challenging, and would involve a high parts count and considerable board space, the concept is attractive if implemented as an IC. One particular single-chip instantiation, the IR1167, needs only three external components. The chip, plus a decoupling capacitor, a gate resistor, and a programming resistor, fits into less than 160 mm2 (0.25 in.2).
For zero-voltage switching (ZVS), the primary is necessary. Otherwise, any reactive power flow between the secondary- and the primary-converter stages reduces overall system efficiency. Traditional transformer sensing requires high current, which causes additional conduction losses. These losses at least partially erode the advantage of primary-ZVS operation. The new control method's gate turn-off falls close to the secondary current's zero crossing, but always on a negative threshold. Therefore, this approach eliminates reactive power flow between the output capacitors and the transformer (Fig. 4b).
This FET-voltage-based approach has only negative drain-to-source-voltage thresholds. As a result, SR drain-to-source current can't flow through the device under any condition. This fact, in turn, reduces the overall power flow and, thereby, losses in the primary circuitry, including switches and magnetics. Figure 5 shows the impact on secondary-current rms value and the consequent reduction in conduction losses on a 120-W, 19-V laptop adapter.
The IR1167 also allows CCM operation without further circuit modification. Although traditional current-transformer-based designs can work in DCM or CrCM, they can?t efficiently work in CCM. The new control method allows operation in all modes, providing the best efficiency and cost-effectiveness.
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