1 Prof. S. Ben-Yaakov , DC-DC Converters [8- 4] Gate Current V S - - PDF document

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• Prof. S. Ben-Yaakov , DC-DC Converters

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Drivers

• Prof. S. Ben-Yaakov , DC-DC Converters

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D L C RL Vo Vin

Driving a MOSFET

• Prof. S. Ben-Yaakov , DC-DC Converters

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Cgs Cgd Ig Cds Rg Vs Q Vgs Vt Qgs+Qgd Qgs+Qgd Miller capacitance 15V Q1 Q2 Q3

Capacitors are non-linear

Driver Requirements

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• Prof. S. Ben-Yaakov , DC-DC Converters

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t t t VS Vgs Ig 15V D L C RL Vo Vin Rg VS Vgs Ig

Gate Current

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 5]

Calculating Required Drive

* Assumption: Constant Current

t t ? ? VS Ig

Cgs Cgd Ig Vs Rg G Vo Rg Ceq G Ig Vs

Method A: Equivalent capacitor

• Prof. S. Ben-Yaakov , DC-DC Converters

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VCgs(0) = 0 ∆ VCgs(∞) =VGSmax VCgd(0) = -VO ∆ VCdg(∞) = VGSmax

gd O GS gs GS total

C ) V V ( C V Q + + = Ig Vo VCgd VCgs ) V V 1 ( C C V Q C

max GS

• gd

gs max GS total eq

+ + = = Miller Effect

max gs total eq

V Q C = Cgs Cgd Ig Vs Rg G Vo

Gate capacitance

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• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 7]

Calculating required Ig (constant), ton:

max gs eq

• n

g total

V C t I Q ⋅ = ⋅ = ) V V 1 ( C C C

max GS

• gd

gs eq

+ + =

• n

max gs eq g

t V C I =

Gate capacitance

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 8]

Calculating Required Drive

Q Vgs Qtotal Vo dependent

Method B: Gate Input Charge

• n

g total

t I Q ⋅ =

• n

total g

t Q I =

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 9]

Gate Drivers

Push pull - complementary Vc=Vgs max O.K. Watch for through-shoot current pnp npn Vc Rg s d s d p channel n channel

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• Prof. S. Ben-Yaakov , DC-DC Converters

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OR n channel n channel Vc s d s d Logic level Gate driver Rg Vc PWM controller

Drivers BAD

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 11]

Commercial driver

Totem pole

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 12]

r G G r

t Q I Q I t ∑ = ∑ = ⋅ Question: Why can IG be considered constant ? tr IG - ? Given: required tr VP IG

Example

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• Prof. S. Ben-Yaakov , DC-DC Converters

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S 10 nS 100 t If

7 r −

= ≈ nQ 50 Q ≈

Σ

A 2 nQ nS 50 100 I = = Significant current !

Example

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 14]

Exercise

V2 TD = 0 TF = 0.01u PW = 10u PER = 20u V1 = 0 TR = 0.01u V2 = 15 Dbreak D1

• ut_gnd

gate L2 {L1*(n*n)} RL {Load} drain V1 {Vin} R4 10meg

• ut

C1 220u IC = 6 K K1 COUPLING = 1 K_Linear L1 = l1 L2 = l2 PARAM ET ERS: n = 0.5 Vin = 12 Load = 10 L1 = 300u

+

• +
• Sbreak

S1 L1 {L1}

Replace the switch with a mosfet, put a 10 Ohm

resistor is series with gate and measure the input current to gate

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 15]

High-Side Drive

The problem Possible solution : Transformer coupling Problem 1.Wide range of Don Vb VGS Gate driver Rs

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• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 16]

Possible solution : Problem #2 - Leakage

? ?

n3 n2 n1 Rg

Like Forward

Lm Llkg Cin Rg

Driver isolation

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 17]

Power and signal pass via transformer –

High Frequency modulation

Transformer isolation

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 18]

Transformer isolation

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• Prof. S. Ben-Yaakov , DC-DC Converters

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Signal coding and reconstruction

Transformer isolation

• Prof. S. Ben-Yaakov , DC-DC Converters

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Solution #2: Optical Solution #3 Potential offset VC - floating supply Driver VC Driver +VC Re

Driver isolation

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 21]

Potential offset + floating C supply

Available for HB, FB Driver Internal circuit in C +15V Rg Re

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• Prof. S. Ben-Yaakov , DC-DC Converters

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HB Driver

• Prof. S. Ben-Yaakov , DC-DC Converters

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HB Driver

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 24]

Low-side drive

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• Prof. S. Ben-Yaakov , DC-DC Converters

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Why the diodes ?

Steering diodes

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 26]

Turn “off”

RG RL CGS LS I VCEsat DRIVER RG RL CGS LS I

• Prof. S. Ben-Yaakov , DC-DC Converters

[8- 27]

Lock current close to source

Trace current path to make sure current is locked

G PG VCC Rf Ω 10 DRIVER

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• Prof. S. Ben-Yaakov , DC-DC Converters

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High Low

Current path

• Prof. S. Ben-Yaakov , DC-DC Converters

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Real life example

UC3823

F 1 . µ Ref VC G PG V ∆ Ref VC G PG VCC VCC