®
TOPSwitch Flyback Power
Supply Efficiency
Application Note AN-19
andconventionalpowersupplydesigns,efficiencymeasurement
techniques, and general design considerations for constructing
efficient flyback power supplies using TOPSwitch. For a more
detailed treatment of flyback power supply and transformer
design refer to application notes AN-16, AN-17 and AN-18.
Introduction
Efficiency should be a major consideration when designing a
flyback switching power supply. It can impact many aspects of
a product design, from enclosure size and ventilation to safety
qualification. Many design choices affect the efficiency of a
givenpowersupplydesign, includingsuchseeminglydisparate
factors as the value of the bulk storage capacitor, transformer
core geometry and construction, choice of output rectifier, and
snubber and clipper networks. This application note will
concentrate on efficiency comparisons between TOPSwitch
TOPSwitch vs. Discrete Design
For a given output power, the efficiency attainable with a
TOPSwitch flyback power supply is equal to or better than a
conventional power supply design using a PWM IC controller
Design Type:
(On Resistance/Voltage Rating)
3842 + MOSFET
(1.2 Ω/600V)
TOPSwitch
(3.6 Ω/700V)
DISCRETE vs. TOPSwitch PERFORMANCE (@ VIN = 120 VAC)
Output Power
Input Power
34.27 W
39.38 W
87%
34.21 W
38.63 W
88.6%
Efficiency
Power Loss
5.11 W
76 KHz
4.42 W
100 KHz
Operating Frequency
POWER LOSS BUDGET
0.37 W
0.16 W
1.07 W
None
MOSFET RDS(ON)
Conduction Loss
Switching Loss
Current Sense Resistor
MOSFET CV2f LOSS
MOSFET Crossover Loss
Start Up Circuit
0.43 W
0.32 W
Negligible
Negligible
0.05 W
0.98 W
1.07 W
1.08 W
0.03 W(1-2 W*)
0.3 W
PWM Controller
Other Losses
1.00 W
Output Diode
1.20 W
Clamp Circuit
Miscellaneous Loss
(Input Filter, Bridge,
0.54 W
0.93 W
Transformer, Output Filter,
Secondary Feedback)
*A more conventional single resistor start-up circuit will dissipate 1-2 Watts.
Figure 1. Power Loss Comparison for Discrete and TOPSwitch Supplies.
June 1996