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LM5037

www.ti.com

SNVS578D – NOVEMBER 2008 – REVISED MAY 2015

Device Functional Modes (continued)

7.4.2 Topology and Control Algorithm Choice

The LM5037 device has all the features required to implement double-ended power converter topologies such as

push-pull, half-bridge and full-bridge with minimum external components. One key feature is the flexibility in

control algorithm selection. For example, the device can be used to implement either voltage mode control or

current mode control. Designers familiar with these topologies recognize that conventionally, current mode

control is used for push-pull and full-bridge topologies while voltage mode control is required for the half-bridge

topology. In limited applications, voltage mode control can be used for push-pull and full-bridge topologies as

well, with special care to maintain flux balance, such as using a dc-blocking capacitor in the primary (full-bridge).

The goal of this section is to illustrate implementation of both current mode control and voltage mode control

using the LM5037 device and aid the designer in the design process.

7.4.3 Voltage Mode Control

a saw-tooth modulation ramp signal shown in Figure 18. The slope of the signal at RAMP varies in proportion to

the input line voltage. The varying slope provides line feed-forward information necessary to improve line

the input voltage (VIN) to stabilize the Volt • Second product of the transformer primary. Using a line feed-forward

ramp for PWM control requires very little change in the voltage regulation loop to compensate for changes in

input voltage, as compared to a fixed slope oscillator ramp. Furthermore, voltage mode control is less susceptible

to noise and does not require leading edge filtering, and is therefore a good choice for wide input range power

converters. Voltage mode control requires a more complicated compensation network, due to the complex-

conjugate poles of the L-C output filter.

In push-pull and full-bridge topologies, any asymmetry in the volt-second product applied to primary in one phase

may not be cancelled by subsequent phase, possibly resulting in a dc current build-up in the transformer, which

pushes the transformer core towards saturation. Special care in the transformer design, such as gapping the

core, or adding ballasting resistance in the primary is required to rectify this imbalance when using voltage mode

control with these topologies. Current mode control naturally corrects for any volt-second asymmetry in the

primary.

width each cycle. The CLK pulse width is same as the dead-time set by RT2. The minimum possible dead-time

(5)

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