Circuit Ideas: High-efficiency dc/dc conversion

Nov. 8, 2007
One of the primary goals when designing dc/dc conversion circuits is to get a high conversion efficiency.

Without careful design, it is easy to dissipate as much energy in the conversion circuit as gets delivered to the load.

The dc/dc conversion method of choice for moderate power output is often a resonant-reset forward converter with synchronous rectification. It is frequently found in isolated industrial and telecom applications. A key component that determines conversion efficiency in these circuits is the resonant capacitor Cres. It facilitates zero-voltage turn-off of a MOSFET switch connected to the primary of an isolation transformer. High efficiency comes from careful choice of the Cres value and use of fast synchronous rectifier drive circuitry. An example is the converter shown in the accompanying diagram which delivers 3.3 V at 30 A from a 48-V source with over 92% efficiency.

The circuit controls the MOSFET switch with a synchronous PWM controller chip, a LTC3723-1. Though this chip can provide push-pull outputs, here only one of its two outputs is used because this is a single-ended supply. Cres must be small enough to let the transformer reset — that is, allow the remnant magnetism to go to zero — during the time available. Reset time is that during which the transformer voltage is not being controlled by the primary switch or the active forward rectifier in the secondary.

There are two inductances of concern for efficiency considerations: the primary inductance Lpri and leakage inductance. The energy in both must be dumped into the effective capacitance across the transformer primary — predominantly Cres. For a typical resonant reset waveform, this requires slightly more than a quarter of the period of the resonant frequency:

fres = 1 /[ 2 π √(LpriCres)],

as the inductor energy is depleted when the capacitor reaches its peak voltage. Allowing approximately half of this period lets the primary switch’s drain voltage return to the input voltage. This, in turn, results in lower turn-on switching loss.

Also note the Cres value determines the peak voltage on the primary and secondary forward switches. Thus, you can use the value of Cres to keep the peak switch voltage below a desired level. This is an opportunity to use a lower-voltage MOSFET(s) with lower switching and/or conduction losses, and it is often overlooked.

Unfortunately, boosting the capacitance beyond that needed for the transformer reset carries the penalty of a longer reset and capacitor return time. Too much reset time can reduce efficiency or force the use of a lower switching frequency.

Use of a synchronous rectifier drive circuit such as the LTC3900, together with appropriate timing, reduces the reset and return time considerably. Effectively you are turning the forward rectifier off at or near the peak of the leakage inductance resonant ring. This starts the primary inductance reset at the peak voltage, rather than at zero.

The accompanying figure shows a typical unoptimized resonant reset waveform: Note the peak voltage of 230 V exceeds the MOSFET’s rating of 200 V. The total reset and return time is 2.0 μsec. The second figure shows the waveform of the optimized reset circuit. The peak voltage is now down to 180 V and the total reset and return time is down to 1.6 μsec. Input power measurements at 48 V in showed a savings of 2.4 W.

Dave Burgoon, Linear Technology Corp.

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The typical voltage waveform at the drain of the primary switch (top) exhibits a peak ringing voltage of 230 V when resonant capacitor Cres is set at 0.5 nF. But a Cres value of 2.2 nF cuts peak voltage to 180 V (bottom). Total transformer reset and return time also drops from 2.0 μsec. to 1.6 μsec.

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