Resonant Boost Converter Design

Justin Burkhart

April 15, 2009

Presentation Outline

• Project Goals

• The Class E Inverter

• Resonant Rectifier

• LDMOS Device Model

• Completed Boost Converter Design

Project Goals

• Design a resonant boost converter using TI LBC5 process LDMOS devices

• Targeted at automotive applications with:– 11-16 Vdc Input– 30 Vdc Output– 10-20 Watts– Highest possible switching frequency with

efficiency >80% (if possible)

The Class E Inverter

0 10 20 30 40 50-0.5

0

0.5

1

1.5

Time (ns)

Cu

rre

nt (

A) i

o(t)

I

0 10 20 30 40 50

0

20

40

Time (ns)

Vo

ltag

e (

Vo

lts) V(t)

Vo(t)

• L1 is large choke with only DC current

• Switch is opened and closed periodically

• Assumed high enough Q such that io is sinusoidal

• When the switch opens, circuit is designed such that the voltage V(t) rings back to zero D*T later thus providing a ZVS opportunity

Adjusted to deliver DC and AC power to the load

The Class E Inverter

0 10 20 30 40 50-0.5

0

0.5

1

1.5

Time (ns)

Cu

rre

nt (

A) i

o(t)

I

0 10 20 30 40 50

0

20

40

Time (ns)

Vo

ltag

e (

Vo

lts) V(t)

Vo(t)

Adjusted to deliver DC and AC power to the load

Pros• Zero Voltage Switching• Only one ground referenced switch required

Cons• High peak device voltage • Large choke inductor• Sensitive to load changes• Limited minimum output power when C1 is constrained

Resonant Input Inductor

This results with:

• Faster Transient Response

• Lower Minimum Output Power

• Flexibility in choice of C1

To see what happens when the value of L1 is reduced break it into two hypothetical inductors, one that carries only DC current and one that carries only AC current.

In this configuration, L1-AC is in parallel with C1

Equivalent Load for Class E Inverter

The load of the Class E Inverter Circuit is tuned to look inductive and can be modeled by an inductor and resistor

This equivalent load provides for a simpler design procedure when the inverter will be used in a DC/DC converter

Solve for component Values3. Solve for the drain voltage by integrating

current in the equivalent C1

4. Solve for the fundamental Fourier component of drain voltage

5. Set drain voltage and slope of drain voltage to zero at switch turn on time

6. Solve for phase of the voltage and current fundamental components

7. Solve for circuit component values

1. Assume DC current in the input

2. Assume sinusoidal current in load

Resonant RectifierTo transform the Class E inverter into a DC/DC boost converter, the output of the inverter must be rectified

A resonant rectifier is used since the losses incurred by a hard switched rectifier at high frequency are too high to maintain good efficiency

The rectifier load is modeled as a constant voltage source since the complete DC/DC converter will use feedback to hold the output voltage constant

Resonant RectifierTo maintain ZVS the rectifier must present the same impedance to the Class E Inverter as the L-R load network

When designing the rectifier, the goal is to choose L and C such that rectifier will have the same current Io(t) as the L-R circuit. This will maintain ZVS in the inverter.

Rectifier Operation

• Diode turns on when Vdiode(t) goes > Vout

• Diode turns off when Io(t) goes < 0

• Initial conditions are known, thus equations for Io(t) and Vdiode(t) can be derived

• Using initial conditions, ton and toff must be solved for

• Closed form solution is not easily arrived at since equations are non-linear and very messy

0 10 20 30 40-20

0

20

40

Time (ns)

Vo

ltag

e (

Vo

lts)

VAC

+VDC

0 10 20 30 40-50

0

50

Time (ns)V

olta

ge

(V

olts

)

Vdiode

(t)

0 10 20 30 40-2

0

2

4

Time (ns)

Cu

rre

nt (

Am

ps)

Io(t)

Vdiode(t)

Rectifier Equivalent ModelRectifier Model 1 Model 2

Total Current: Solid Lines DC Current: Dashed Lines

0 5 10 15 20 25 30 35 40

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time (ns)

Cu

rren

t (A

mp

s)

Tune Characteristic Impedance

0 5 10 15 20 25 30 35 40-1

-0.5

0

0.5

1

1.5

2

Time (ns)

Cu

rren

t (A

mp

s)

Tune Resonant Frequency

Example Of Rectifier Tuning

Increase Fo

Decrease Zo

Goal:

• Adjust the resonant frequency of the rectifier until the input current has the desired phase

• Adjust the characteristic impedance of the rectifier until the desired output power is reached

20 25 30 35 4010

12

14

16

18

20

22

24

Characteristic Impedance (Ohms)

Po

wer

Ou

tpu

t (W

att

s)

LDMOS Device ModelTypical LDMOS Device Model Simplified LDMOS Device Model

Parasitic capacitance measurement procedure

Device Measurement DataRds-on Data

• Test Device only has 1 bond wire per device terminal

• This is a limiting factor in measuring small parasitic device resistances

• TI process engineers report that Rds-on for devices in this lot were measured at 165mOhm

• This would result with an estimate of about 475mOhm bond wire resistance

Device Measurement Results

Coss Data Ciss Data

1.45 pF extra parasitic capacitance from measurement

2.3 pF extra parasitic capacitance from measurement

Simulated DC/DC Converter

Converter Efficiency

Efficiency and Output Power Loss Breakdown by Component

Gate Drive is Not Included

11 12 13 14 15 16 17 1812

14

16

18

20

22

24

Po

wer

Ou

t (W

atts

)

Vin (Volts)11 12 13 14 15 16 17 18

83

83.5

84

84.5

85

85.5

86

Eff

icie

ncy

(%

)

Future Work

• Device optimization

• Parametric variance analysis

• Gate driver

• Power section prototype

• Integrated controller design

• Complete converter