Texas Instruments UCC2891 User Manual
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EVM Test Setup

High efficiency is achieved using self-drivensynchronous rectification on the secondary side. Q3 and Q4 are placed in parallel and make up the forward synchronous rectifier (SR), while the reverse SR is made up of the parallel combination of Q5, Q7 and Q8. If the duty cycle were limited to 50% then the reverse SR could be reduced to only two parallel MOSFETs, but since these devices are operating near 60% duty cycle during the freewheel mode, they carry a higher average current than seen by Q3 and Q4. The output inductor L1 has a coupled secondary, referenced to the primary side, used to provide bootstrapping voltage to U1. A stable bias for the optocoupler, U2 is provided by the series pass regulator made up of D6, Q6 and some associated filtering.

Scope jacks J2 and J3 allow the user to measure the gate-to-sourceanddrain-to-sourcesignals for Q2, the primary MOSFET. J4 and J5 allow convenient access to the gate drive signals of each SR on the secondary side. J6 and J7 are available allowing the option of using a network analyzer tonon-invasivelymeasure the control to output loop gain and phase.

5EVM Test Setup

Figure 2 shows the basic test set up recommended to evaluate the UCC2891EVM.

 

 

 

 

 

 

V2

 

 

 

 

 

 

 

+

-

 

 

 

 

 

 

 

 

 

+VOUT

 

 

 

 

 

J9

 

J10

-

 

 

 

 

 

+Vout

 

-Vout

-

 

 

 

+VIN

 

 

 

 

 

+

 

 

-

 

 

 

 

 

 

 

 

 

 

J1

 

 

 

J8

 

 

+

 

-

 

 

+

 

 

 

 

 

 

+

V1

VIN

 

 

 

 

+

-

-VIN

 

 

 

 

 

 

 

 

 

 

 

 

 

V-Clamp

 

 

 

-VOUT

 

 

 

 

 

 

 

 

 

 

 

J3

 

 

 

SR-QFGate

 

 

 

A1

 

 

 

 

 

 

 

 

 

 

 

J4

 

 

 

 

 

 

 

 

 

 

 

 

Q2 Gate

 

 

 

J5

 

 

 

 

 

 

 

 

 

 

 

 

J2

 

 

 

SR-QRGate

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

J6 Loop+

 

 

 

 

 

 

Texas Instruments

J7 Loop-

 

 

 

 

DANGER HIGH VOLTAGE

 

 

 

 

 

HPA034

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UCC2891 Active Clamp Converter

 

 

 

 

 

 

 

FAN

 

 

LOAD1

3.3V/30A

Figure 2. Recommended EVM Test Configuration

SLUU178A –November2003–RevisedDecember 2006

Using the UCC2891 Active Clamp Current Mode PWM Controller

7

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EVM Test Setup

5.1Output Load (LOAD1)

For the output load to VOUT, a programmable electronic load set to constant current mode and capable of

sinking between 0ADC and 30ADC, is used. Using a dc voltmeter, V2, it is also advised to make all output voltage measurements directly at J9 and J10 pins. Unless the load has remote sense capability, measuring VOUT at LOAD1 results in some voltage measurement error, especially at higher load current, due to finite voltage drops across the wires between J8 and the electronic load.

5.2DC Input Source (VIN)

The input voltage is a variable DC source capable of supplying between 0 VDC and 72 VDC at no less than 3.5 ADC, and connected to J1 and A1 as shown inFigure 2. For fault protection to the EVM, good common practice is to limit the source current to no more than 4 ADC for a 36 V input. A dc ammeter, A1 should also be inserted between VIN and J1 as shown inFigure 2.

5.3Network Analyzer

A network analyzer can be connected directly to J6 and J7. The UCC2891EVM provides a 51.1-Ω resistor (R25) between the output and the voltage feedback to allow easynon-invasivemeasurement of the control to output loop response.

5.4Recommended Wire Guage

The connection between the source voltage, VIN and J1 of the EVM can carry as much as 3.25ADC. The minimum recommended wire size is AWG #20 with the total length of wire less than 8 feet (4 feet input, 4

feet return). The connection between J8 of the EVM and LOAD1 can carry as much as 30ADC. The minimum recommended wire size is AWG #16, with the total length of wire less than 8 feet (4 feet output, 4 feet return).

5.5Fan

Most power converters include components that can be hot to the touch when approaching temperatures of 605C. Because this EVM is not enclosed to allow probing of circuit nodes, a small fan capable of 200-400LFM is recommended to reduce component temperatures when operating at or above 50% maximum rated load current.

8

Using the UCC2891 Active Clamp Current Mode PWM Controller

SLUU178A –November2003–RevisedDecember 2006

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