STCMB1 datasheet(38/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

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DocID028958 Rev 4

Figure 21. Improved burst mode waveforms

7.21

HB: adaptive deadtime (ADT)

A deadtime TD is inserted between the turn-off of either switch and the turn-on of the

complementary one, where both switches are in the OFF state. This is essential to achieve

soft-switching: its value has to be larger than the time TT needed for the rail-to-rail swing of

the half-bridge leg midpoint (HB). This duration TT depends on the total parasitic

capacitance of the node HB (CHB) that has to be completely charged or depleted and on

the value of the resonant tank current during the transition.

With good approximation, the tank current during the transition time TT can be considered

constant and equal to the “switched current” IS, e.g.: the value of the tank current as the

transition begins. If CHB denotes the total parasitic capacitance of the half-bridge midpoint (it

includes the Coss of the MOSFETs, the transformer's primary winding parasitic capacitance,

plus other stray contributors), the condition for soft-switching is:

Equation 23

which should be met under all operating conditions. This suggests that TD should be large

enough to always exceed TT, especially with maximum Vin and at no load, where IS is at

a minimum and TT at a maximum. However, fixing a too long deadtime may lead to losing

soft-switching too: in fact, the tank current must not change sign within the deadtime, which

could lead to turn-on either the MOSFET with a non-zero drain-to-source voltage or, even

worse, with the body diode of the other MOSFET conducting (see Section 7.22: HB: hard

switching prevention (HSP) and anti-capacitive mode protection (ACP) on page 41 for more

details). This might occur at the maximum load and minimum Vin, especially when the tank

circuit is designed for a low magnetizing current to optimize light-load efficiency. Additionally,

a too long deadtime might increase conduction losses in the body diodes and significantly

limit the operating frequency of the half-bridge.

A good approach is to automatically adjust TD so that it tracks TT, keeping TD TT under all

operating conditions. This is the objective of the adaptive deadtime function in the STCMB1.

Figure 23 shows the principle schematic and its key waveforms. An edge detector (the |d/dt|

block) senses that the node HB (connected to the OUT pin) is swinging from B+ to ground or

vice versa. The output of the |d/dt| block is high as long as the OUT pin is swinging and, as

i n

H B

≤

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