AN2043

AN2043
Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 2043
Keywords: power converter, power supply, isolated power supply, current-mode, PWM, synchronous
rectifier, switch mode, fixed frequency, step down, forward converter
APPLICATION NOTE 2043
A Compact, Isolated Power Supply, 48V to 3.3V at
7A
By: Suresh Hariharan, Director and Product Definer
May 21, 2003
Abstract: This application notes describes a compact isolated switch-mode power supply that is capable
of providing a 7A 3.3V output from a 40VDC–60VDC input at an efficiency of 83%–90%. The converter
employs a MAX5052A current-mode PWM controller with all of the circuitry required to provide
synchronous rectification, over-current and short-circuit protection, remote sense, and primary-secondary
isolation of 1500VDC. The total volume occupied is only 1.02 cubic inch. A materials list and suggested
layout are provided.
General Description
The MAX5052A is a current-mode PWM controller. It contains all the necessary control circuitry required
for the design of an isolated power supply. This application note describes the design of a 23W isolated
power supply. The topology employed is a single transistor forward with synchronous rectification.
Remote sense pins compensate for load cable voltage drops. The switching frequency of the MAX5052A
is set at 262kHz and it is internally trimmed to ±12% accuracy. The MAX5052A has a maximum duty
cycle of 50%.
Specifications
Input voltage 40VDC–60VDC
Power supply dimensions: 1.79" × 0.95" × 0.60"
Output voltage: 3.3V ±2%
Output current: 7A
Over-voltage 4V ±5%
Over-current and short circuit protection
Remote sense
No damage should occur to the remote sense resistors if the connections of the remote sense is
reversed.
Isolation voltage, primary to secondary 1500VDC.
Circuit Operation
The input voltage is connected to the connector J2. C8 is a 1µF, 100V high frequency Ceramic
Page 1 of 9
capacitor, connected across the input. An external electrolytic capacitor is connected across the input of
the power supply. Resistor R6 charges up the VIN bypass capacitor C2 from the input. C2 is a 4.7µF,
50V electrolytic capacitor. Once the converter starts up, capacitor C2 is bootstrapped by the bias
winding. The voltage across R5 is a pulsating waveform, with a peak voltage equal to the input voltage
times the turns ratio of the bias winding to the primary winding. Resistor R1 and capacitor C14 average
this pulsating waveform. The diode D8 is used to prevent the capacitor C2 from discharging into the
series combination of resistors R1 and R5. The bootstrapped winding voltage across C14 is proportional
to the output voltage. The MOSFET Q3 is the primary switching FET. R14 is the current sense resistor.
Resistor R7 and R12 form the under-voltage lockout set point of the power supply. With the given
values, the turn-on voltage is 39V.
Synchronous rectification on the secondary side is used to increase the power supply efficiency. The
gate charge retention scheme is used to guarantee gate drive voltage on the freewheeling MOSFET
when the transformer winding voltage has gone to zero. In a dc-dc converter with a two-to-one line
swing of the input voltage, this time can be 50% of the duty cycle, or higher, at the highest input voltage.
This is true in the case where the reset winding has the same number of turns as the primary power
winding and the reset winding is returned to the input via a reset diode. In the current application the
input voltage was changing from 40V to 60V.
The forward path synchronous MOSFET Q1 is turned on when the transformer voltage on the secondary
goes positive and is turned off when the main switching FET is turned off. The freewheeling MOSFET
Q4 is turned on when switch Q3 is turned off and the transformer voltage reverses. Once the
transformer voltage goes to zero, MOSFET Q2 is kept on because there is no discharge path on the
gate of Q2. When the primary switching FET Q3 is turned on, the MOSFET Q4 is turned on and this
discharges Q2 rapidly resulting in the turn of off Q4.
U1 is the secondary feedback opto-coupler that sends the feedback signal across the isolation barrier.
U5 is an opto-triac which shuts off the PWM controller in case of an over-voltage by discharging C2 and
keeping it discharged until the input power is recycled. R23 and R18 are the positive and negative
remote sense resistors. When the power supply hits the current limit the duty cycle will pitch back,
causing the output voltage to drop. This will also cause the voltage across C14 to drop. Since the voltage
across C14 is directly proportional to the output voltage, a heavy overload on the output would cause a
sufficient drop in the bias voltage on C14 such that the voltage on C2 would drop below 9.9V causing
the controller to shut down and go into a hiccup mode of operation, thus causing a reduction in the RMS
short circuit current. This mode of operation of the power supply allows us to short the power supply
output without overheating the output semiconductors and inductor. Diodes D9 and D10 provide an
alternate path for the current when the remote sense wires are crossed. This will cause the output to be
short-circuited by diodes D9 and D10 and will put the power supply into the hiccup mode of operation.
This prevent the resistors R18 and R23 from opening up due to excessive current.
Performance Data
Peak-to-peak ripple at an input voltage of 60V. Output 3.3V at 7A
Measured peak-to-peak ripple was 124mV at 20MHz bandwidth.
Page 2 of 9
Figure 1.
Peak-to-peak ripple at an input voltage of 48V. Output 3.3V at 7A
Measured peak-to-peak ripple was 100mV with the bandwidth set at 20MHz.
Figure 2.
The output ripple and noise could be further improved by increasing the number of output capacitors or
by using output capacitors with lower ESR.
Page 3 of 9
Transient Response
Transient response measured at 48V input and a load step decrease from 5.25A to 3.5A.
The peak positive excursion was measured at 160mV.
Figure 3.
Transient response measured at 48V input and a load step increase from 3.5A to 5.25A.
Here the peak negative excursion was 220mV. The difference in behavior can be attributed to the
change in crossover frequency and the phase margin at 3.5A and 5.25A.
Page 4 of 9
Figure 4.
Table 1. Efficiency Data
IIN
VIN
VOUT
IOUT
Efficiency
40V
0.32352A
3.2685V
3.5006A
88.4%
40V
0.45893A
3.2675V
5.0111A
89.2%
40V
0.6485A
3.2653V
7.0111A
88.25%
48.1V
0.2770A
3.2663V
3.5332A
86.4%
48.1V
0.38655A
3.2663V
5.0116A
88.0%
48.1V
0.54735A
3.26652V
7.044A
87.6%
60V
0.22912A
3.2657V
3.5010A
83.2%
60V
0.31670A
3.2657V
5.0116A
86.1%
3.2647V
7.011A
86.4%
60V
0.44163A
Note: All data taken at 25° C.
PCB Information
The PCB is compact and it has components on both sides. The dimensions along with the component
placement are shown below.
Page 5 of 9
Figure 5.
Figure 6.
Table 2. Component List for 23W Power Supply
Ref Des
Qty.
Description
Page 6 of 9
C1
1
Ceramic capacitor, 1000pf, 25V, X7R (0402)
C2
1
Electrolytic capacitor, 4.7µF, 50V
Panasonic EEVFK1H4R7R
C3
1
SP capacitor, 180µF, 6.3V
Panasonic EEFUEOJ181R
C4, C5
2
Ceramic capacitor, 4.7µF, X5R, 6.3V (0805)
C6
1
Ceramic capacitor, 680pF, 50V, X7R (0603)
C7
1
Ceramic capacitor, 4700pF, 250vac, X7R(1812)
Murata GA243DR7E2472MW01L
C8
1
Ceramic capacitor, 1µF, 100V, X7R (1812)
C9
1
Ceramic capacitor, 1µF, 16V, X7R (0805)
C10
1
Ceramic capacitor, 100pF, 50V, COG (0402)
C11, C12
2
Ceramic capacitor, 0.47µF, 10V, X5R (0603)
C13
1
Ceramic capacitor, 1µF, 10V, X5R (0603)
C14
1
Ceramic capacitor, 0.1µF, 50V, X7R (0603)
C15
1
Ceramic capacitor, 4700pf, 50V, X7R (0402)
C16
1
Ceramic capacitor, 0.047µF, 16V, X7R (0402)
R1
1
Resistor 220Ω, 5%, (0603)
R2
1
Resistor 4.7Ω, 5% (0603)
R3, R11
2
Resistor 3.30kΩ, 1% (0402)
R4
1
Resistor 10Ω, 5% (0805)
R5
1
Resistor 3.3kΩ, 5% (1206)
R6
1
Resistor 82kΩ, 1% (0805)
R7
1
Resistor 274kΩ, 1% (0603)
R8
1
Resistor 221Ω, 1% (0603)
R9
1
Resistor 4.99kΩ, 1% (0402)
R10, R13
2
Resistor 1kΩ, 1% (0402)
R12
1
Resistor 10kΩ, 1% (0402)
R14
1
Resistor 0.15Ω, 1% (1206)
R15
1
Resistor 36kΩ, 1% (0402)
R16
1
Resistor 330Ω, 1% (0402)
R17
1
Resistor 205Ω, 1% (0402)
R18, R23
2
Resistor 10Ω, 5% (0402)
R19
1
Resistor 124Ω, 1% (0402)
R20
1
Resistor 16.2kΩ, 1% (0402)
R22
1
Resistor 100Ω, 5% (0402)
R24
1
Resistor 22Ω, 5% (0402)
R25
1
Resistor 2.2kΩ, 5% (0402)
R26
1
Resistor 1MΩ, 5% (0402)
R27
1
Resistor 10Ω, 5% (0603)
Page 7 of 9
R28
1
Resistor 68Ω, 5% (0402)
T1
1
Power transformer, Transpower TTI8619
Q1, Q2
2
N-channel MOSFET 20V (DPAK)
Vishay SUD70N02-09
Q3
1
N-channel MOSFET 150V (DPAK)
Fairchild Semi FDD120AN15A0
Q4
1
N-channel MOSFET 30V, SOT23
Fairchild NDS351AN
L1
1
Choke 4.7UH,
Coilcraft DR0810-472
D1, D5
2
Diode 200V, 200mA
Panasonic MA115CT
D2
1
Signal diode 100mA 75V
Diodes Inc 1N4148W
D3
1
Schottky diode 8A, 35V (PowerMite)
Diodes Inc SBM835L
D4, D6, D7, D11,
D12
5
Schottky diode 30V, 100mA
Panasonic MA2S784
D8
1
Diode 100V, 200mA
Panasonic MA111CT
D9, D10
2
Schottky diode, 3A, 20V
Diodes Inc B320A
U1
1
Optocoupler
Fairchild HMHA2801
U2
1
Current mode PWM
Maxim MAX5052AEUA
U3, U4
2
Precision reference 1.25V, 1% (SOT23-5)
Texas Instruments TLV431ACDBVR
U5
1
Opto-coupler triac
Fairchild MOC3023
For Larger Image
Figure 7. Schematic of 23W power supply.
Related Parts
MAX5052A
Current-Mode PWM Controllers with an Error Amplifier for
Free Samples Page 8 of 9
Isolated/Nonisolated Power Supplies
More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 2043: http://www.maximintegrated.com/an2043
APPLICATION NOTE 2043, AN2043, AN 2043, APP2043, Appnote2043, Appnote 2043
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
Page 9 of 9
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement