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Forward Converter Topology Design Yields Efficiency

A modified forward converter scheme boasts high efficiency and zero "turn off" losses. Snubber circuitry reduces “turn on” and minimizes EMI effects.

MOSHE DOMB, RESEARCH/DEVELOPMENT DIRECTOR, CHEROKEE INTERNATIONAL

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Sometimes crafting an efficient power conversion technology is a mixture of art and science. In an effort to strike performance “gold”, engineers at Cherokee evaluated various topologies to determine the best performing DC-DC converter topology. Their goal: to provide an approach yielding the highest efficiency yet robust and low-component count topology, and very fast yet very stable step load response. After a detailed evaluation, the chosen topology was a modified single-ended forward converter with a non-dissipative turn-off snubber circuit.

This modified forward converter operates at a duty cycle of 65% at low line at 33V, and as a result, has a very low primary current—a high turns ratio in transformer. It also has zero turn-off losses. The turn-on losses are low as well, due to the fact that the primary current is low and the leakage energy is partially recycled in a non-dissipative way and partially returned to the input voltage. One nice feature of this snubber circuit is that the transformer leakage, up to a point, “acts as your ally” in the sense that it reduces the turn-on losses and minimizes EMI effects as well. That’s helpful in military applications where EMI control requirements are stringent.

Eliminates Turn-Off Losses

The forward converter with a non-dissipative turn-off snubber is a topology that eliminates the turn-off losses by charging a capacitor with the magnetic energy—stored in the magnetizing and leakage inductances of the transformer—during the “off” period. During the “on” period, this charge is resonating and hence reverses the polarity across the capacitor and provides zero turn-off losses during the onset of the primary MosFET “on” period.

The non-dissipative snubber enables a duty cycle of 65% thus, the turns ratio of the transformer is very high and the primary current is very low. Combining a 65% duty cycle and almost zero primary switching losses renders a very efficient topology, yet very simple with a low component count.

Snubber Operation in the “Off” Period

Figure 1 depicts the forward converter where D1, CS, LS and D2 constitutes the non-dissipative snubber circuit. During the “off” period of the primary MosFET QP, the capacitor CS is charged by the primary magnetizing and leakage energy and then discharges to +Vin. The charge and discharge of Cs during the “off” period is happening through four time intervals between t0 and t4. The analysis of each interval is shown in Figures 2a, 2b, 2c and 2d.

 

Design Process

The design process is composed of two parts. The first part is a calculation of the turns ratio, which yields 65% duty at Vin of 33V. The second part is a calculation of the snubber values Lm and CS. From the need to reset the transformer at low line, 33V, we demand that the resonance between LM and CS take only one third of the switching cycle.

The snubber circuit performs two functions: the first is to reset the transformer, which actually works in the third quadrant, as shown in Figure 1, and is better usage of the transformer. The second function is eliminating the turn off losses as the primary drain to source voltage slowly ramps up from zero to a peak voltage. The turns ratio is designed for a 65% duty cycle at low line. The transformer magnetizing current is designed to resonate with the snubber capacitor to zero current for all input voltages.

Design Example: 1.8V, 50A Brick

The efficiency was measured for 1.8V, 50A, ¼ brick cycle DC-DC and is shown in Figure 3. For a Vin of 48V and 50A the efficiency is about 90%. A step load was taken from 75% to 100% of 50A. Figures 4 and 5 depict the load step for measured and simulated data accordingly. Overall, the simulation data matches very well with the measured one, which enabled the optimization of the loop response, and also controls the primary drain source peak voltage to have adequate safety margin.

The single-ended forward converter with non-dissipative snubber was successfully implemented achieving high efficiency—90% for 1.8V at 50A—, fast transient (converges within 80 microseconds) and low noise (less then 60mV). All that was done using of a simple low component count and an extremely reliable topology. An example Cherokee product designed with this topology is shown on page 25.

Cherokee International
Tustin, CA.
(714) 544-6665.
[www.cherokeellc.com].

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