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SMPS AC/DC Reference Design User’s Guide
DS70320B-page 42 © 2008 Microchip Technology Inc.
EQUATION 2-28:
The nominal duty cycle is 0.42 ignoring stray voltage drops and inductor current ripple.
This equates to a high-side MOSFET on-time, t
on
, of 840 ns. Therefore, the high-side
MOSFET is rated for 14.9 Arms and the low-side MOSFET is rated for 17.5A.
The selected MOSFETs are International Rectifier IRLR7833PBF, 30V, 4.5 mΩ devices
in a DPAK package, and are mounted directly on the PCB. The intrinsic reverse diode
of the MOSFET is relatively slow in switching, so a fast Schottky diode is placed in
parallel across the MOSFET to reduce switching loss. The conduction losses of the
high-side and low-side MOSFETs are estimated at 0.85W and 1.2W respectively. It is
possible in some designs to optimize the efficiency performance by selecting different
devices for the high-side and low-side switches.
The switching loss in the MOSFETs can be estimated by assuming ideal linear
switching transitions using Equation 2-29.
EQUATION 2-29:
The estimated switching transition time is 50 ns, so the switching loss for each device
is 2.4W.
The gate drive circuitry is a dual Microchip MCP1404 gate-drive IC, which drives the
MOSFET gates directly. The maximum gate threshold of each MOSFET is 4V, so the
drive circuit for the high-side MOSFET is provided by the auxiliary SMPS power rail,
which is higher than the gate threshold and source voltage combined. The PWM
module pin of the dsPIC DSC device interfaces with the gate-drive IC via an inverting
open-collector transistor stage, which provides immunity against ground bounce.
2.3.1.2 OUTPUT CHOKES
The ripple current is given by the relationship shown in Equation 2-30.
EQUATION 2-30:
The ripple current needs to be less than 25% of the output current so each output choke
is a 1 μH Coilcraft SER1360-102KL surface mount. A smaller inductor would improve
the output transient response, but at the expense of higher ripple current and,
consequently, higher output voltage ripple. The DC resistance is 2.5 mΩ and the power
loss is 1.3W for the maximum 23A output current. The peak-to-peak ripple current is
5.9A.
2.3.1.3 OUTPUT CAPACITOR
The output capacitor ripple current is fairly low in a Buck converter due to the
continuous inductor current, and is only dependent on the amplitude of ripple current
in the inductor. The relationship for capacitor rms current is given by Equation 2-31.
1
low o
iI D
=
−
%
1
6
s
wbusorsw
P
VItf
=
(
)
0
on bus
tV V
i
L
−
Δ=
- SMPS AC/DC Reference Design 1
- User’s Guide 1
- Table of Contents 3
- Appendix B. Test Results 4
- DOCUMENTATION CONVENTIONS 7
- WARRANTY REGISTRATION 8
- RECOMMENDED READING 8
- THE MICROCHIP WEB SITE 8
- CUSTOMER SUPPORT 9
- DOCUMENT REVISION HISTORY 9
- SMPS AC/DC REFERENCE 11
- DESIGN USER’S GUIDE 11
- Chapter 1. Introduction 11
- 1.2 BLOCK DIAGRAM 12
- Introduction 13
- HV Bias Supply 24
- CONVERTERS 28
- Chapter 2. Hardware Design 29
- 2.1.1 Power-Train Design 30
- Hardware Design 31
- 2.2 FULL-BRIDGE ZVT CONVERTER 34
- 41.592.74 38
- Chapter 3. Software Design 55
- Software Design 57
- 3.5.1 Output Sequencing 68
- Table 3- 7 70
- Chapter 4. System Operation 71
- System Operation 73
- 4.1.4 System Connections 74
- 4.2 SYSTEM OPERATION 76
- Board Layouts and Schematics 81
- Test Results 101
- AC and 220 VAC 106
- AC WITH 106
- B.8 TEST RESULTS TABLE 108
- Appendix C. References 113
- Worldwide Sales and Service 118
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