NCP1562-100WGEVB ON Semiconductor

NCP1562-100WGEVB NCP1562 100 W 48 V DC-DC Converter Evaluation Board User s Manual NCP1562100WGEVB DESIGN SPECIFICATIONS Active Clamp Forward Topology The flexibility of the NCP1562 is demonstrated by The active clamp forward (ACF) topology has multiple examining a detailed design of a dcdc converter for the advantages compared to a traditional forward converter. The telecom system. The converter delivers up to 100 W at benefits of the active clamp topology can be easily 3.3 V. The converter specifications are listed in Table 1. A forward active clamp topology is selected for the converter, maximized once the unique characteristics of this topology are fully understood. Figure 2 shows a simplified schematic as it provides very high efficiency. of an active clamp forward topology. The transformer model Table 1. DESIGN SPECIFICATIONS (TX1) consists of an ideal transformer, magnetizing (L ) MAG and leakage (L ) inductances. Parameter Symbol Min Max LKG The active clamp network consists of the P-channel clamp Input Voltage V (V) 33 76 in switch (Q ) and clamp capacitor (C ). This clamp clamp Frequency f (kHz) 350 (typ) SW configuration is known as low side active clamp. A high side Full Load Efficiency 90 (%) clamp could have been implemented using an N-channel Duty Ratio D 65% MOSFET and clamp capacitor in parallel with the Output Voltage V (V) 3.267 3.333 transformer primary. However, it requires a floating gate out drive signal increasing system cost and complexity. Output Voltage Ripple V (mV) 50 out(rip) Output Current I (A) 3 30 out Output Power P (W) 100 out Ambient Temperature T (C) 50 A Derating Factor 90% TX1 N :N = 1:N P S I I /N SEC SEC L out I MAG Synchronous + Rectifier V Q in FW L Control C MAG out I P L LKG Q REC NCP1562 OUT1 C C drain clamp OUT2 Q main Q clamp C C Figure 2. Active Clamp Forward Converter The differences between traditional and active clamp Reset Winding forward converters are during the main switch off time. In the active clamp topology, the transformer is reset at a lower voltage during the complete off time instead of a higher Active voltage during a shorter period of time. Figure 3 shows a Clamp comparison between the drain waveforms of both topologies. The traditional forward waveform was taken with a primary:reset winding ratio of 5:3 instead of 1:1. The 5:3 ratio allowed operation above 50% duty cycle. Figure 3. Drain Voltage Waveforms for Traditional and Active Clamp Forward Topologies