Resonant Converters Introduction (1)

1. Resonant ConvertersIntroduction (1) • Very large research area in Power Electronics • Many ideas still in academia, but transfer to industry is being made • Basic motivation is to reduce the switching loss in converters and to allow higher switching frequencies to be obtained • Generally, higher switching frequency is desirable • Smaller filter components (in SMPS for example) • Better quality waveforms Review of conventional “Hard” switching • Power loss due to switching = fS*V*(IONtON+IOFFtOFF)/2 • Places an upper limit on fS because of device heating • Can use snubbers - but just moves loss elsewhere • Another disadvantage of “hard switching” is the electromagnetic interference (EMI) generated

2. Resonant ConvertersIntroduction (2) • Resonant converter techniques attempt to eliminate switching loss by using the properties of resonant circuits to arrange for V or I (or both) to be zero at the instant of switching • Very high operating frequencies have been reported - eg small power supply operating at 15MHz! • Hundreds of different topologies have been proposed which fall into 3 broad categories • Load Resonant - load forms part of a resonant circuit (see Mohan for examples) • Resonant Link Converters - not discussed further here (see Mohan for examples) • Resonant Switch Converters - Modifications of conventional circuits with a resonant circuit associated with each switch to give Zero Voltage (ZVS) or Zero Current (ZCS) switching • Load resonant and resonant switch circuits are the most often exploited • We will consider one resonant switch circuit in more detail - DC to DC forward converter with zero voltage switching

3. A Q Lf VO Cf D LOAD B Basic Forward Converter Circuit ZVS Forward Converter- circuit development A Q Lf L D2 VO Cf D C LOAD B Basic ZVS Forward Converter Circuit

4. A Q Lf L Io iL i D2 VO Cf D C LOAD VC B Basic ZVS Forward Converter Circuit A Q L i D2 IO E VAB D C VC B Circuit for analysis ZVS Forward Converter- assumptions for analysis • Assume ideal devices • Assume iL smooth so that iL= IO • VO = mean value of VAB

5. A L i IO E VAB C VC B Equivalent Circuit for Mode 1 ZVS Forward ConverterMode 1 • Before Mode 1 starts Q has been on for some time (see Mode 5) • Mode 1 starts when Q is turned OFF • Conditions at start of Mode 1: VC = 0, i = IO • IO charges C linearly  VC rises linearly • VAB = E – VC when VC reaches E, VAB tries to go negative and D turns ON  this ends MODE 1 • Duration for Mode 1 is given by:

6. A L i IO E D VAB C VC B Equivalent Circuit for Mode 2 ZVS Forward ConverterMode 2 • Conditions at start of Mode 2: VC = E, i = IO, D is ON • L and C form a resonant circuit (see separate analysis sheet) • VAB = 0, VC follows an offset sine, i follows a cosine:

7. A L i IO E D VAB C VC B Equivalent Circuit for Mode 2 ZVS Forward ConverterMode 2 continued D2 • Mode 2 finishes when VC tries to go negative and D2 turns ON • Duration for Mode 2 (2) can be found from: • Value of i (=I’) at the end of Mode 2 is given by: • I’ is negative

8. A Q L i D2 IO E D VAB C VC B Equivalent Circuit for Mode 3 ZVS Forward ConverterMode 3 • Conditions at start of Mode 3: VC = 0, i = I’ (negative), D is ON, D2 is ON • Resonance stops since C is shorted by D2 • VAB = 0, VC = 0 • Voltage across L = E  i increases linearly (towards 0) • Q has a drive (Gate) signal applied during Mode 3, but doesn’t conduct immediately since i is negative • Mode 3 finishes when i reaches 0 and Q takes over i from D2 • Duration for Mode 3 is given by:

9. A Q L i IO E D VAB C VC B Equivalent Circuit for Mode 4 ZVS Forward ConverterMode 4 • Mode 4 starts when Q takes over i from D2 as it passes through 0 • Conditions at start of Mode 4: VC = 0, i = 0, D is ON, Q is ON • No resonance since C is shorted by Q • VAB = 0, VC = 0 • Voltage across L = E  i increases linearly (towards IO) • Mode 4 finishes when i reaches IO and all of the load current is now flowing through Q  D turns OFF • Duration for Mode 4 is given by:

10. A Q L i IO E VAB C VC B Equivalent Circuit for Mode 5 ZVS Forward ConverterMode 5 • Mode 5 starts when D turns OFF since Q has all the load current • Conditions at start of Mode 5: VC = 0, i = IO, D is OFF, Q is ON, D2 is OFF • No resonance since C is shorted by Q • Circuit is in steady state: VAB = E, VC = 0, i = IO • Mode 5 finishes when Q is turned OFF by the control circuit  back to Mode 1 • Duration for Mode 5 is controllable and provides the means for controlling the output voltage Vo (= mean value of VAB)

11. Blank for ZVS Waveform

12. ZVS Forward Converter Observations • At the instant at which Q turns OFF, the voltage across it is held at virtually zero by C  No turn-off loss (ZVS) • At the instant at which Q starts to conduct (end of Mode3), the voltage across it and the current through it are both zero  No turn-on loss (ZVS and ZCS) • ZVS at turn-on is only possible if V  E • V  IO  circuit must be designed for lowest IO • If the range of IO is large, V will be much larger than E at the maximum current • Q will see a very large voltage when it is off. This is a disadvantage of this particular circuit • Can be partially overcome at the expense of more complexity • To control VO, we can only control the time for Mode 5 (5) • 1  4 are set by the circuit parameters and the output current • Control is more complex than for normal (non ZVS) circuit since both the duty cycle and operating frequency must be varied • Output voltage is given by:

13. Resonant Converters General Observations • ADVANTAGES • Virtually zero switching loss • High operating frequencies possible • High efficiencies • Soft switching - reduced EMI • DISADVATAGES • More components • More complex control • Increased voltage and/or current stress on devices • Possibility of increased conduction loss (can partially outweigh reduction in switching loss)