AN3276 datasheet(13/22 Pages) STMICROELECTRONICS

13 / 22 page

AN3276

The new ST solution - BC2: energy recovery circuit

Doc ID 17975 Rev 1

13/22

2.3

Electrical voltage stress in BC2

Table 1 summarizes the maximum voltage across each semiconductor versus the phases.

The BC2 circuit needs to use a specific diode with a breakdown voltage higher than 600 V.

Moreover, its recovery current should be optimized to avoid an higher current in the power

transistor during the phase [t1-t2]. ST has developed specific diodes (STTH16BC065C,

STTH10BC065C and STTH8BC065) for the BC2 circuit. They have been designed to

sustain average currents of 5 to 8 A (depending on the application) and a repetitive reverse

voltage VRRM of 650 V.

2.4

Calculation of m2 and m1 ratios

To respect the discontinuous operating mode during the timing phases [t1-t2] and [t3-t4], the

time td1 and td2 shown in Figure 7 should be always positive. According to the typical CCM

PFC rules and both tD1_ON and tD2_ON expressions, it becomes easy to define the m1 and

m2 transformer ratio conditions.

and

Where PIN is the input PFC power, Fs is the switching frequency, VmainsRMSmax is the

maximum rms voltage range and the IRMmax is the maximum current recovery under the

turn-on dI/dt at its maximum operating junction condition.

Table 1.

Maximum reverse voltage in the BC2

Maximum

voltages

Phases

Waveform expression

> VOUT?

VRTRmax

t3 - t4

VOUT

VRDBmax

t1 - t2

YES

VRD1max

t0 - t1

t3 - t4

VRD2max

t0 - t1

t1 - t2

t2 - t3

VOUT

OUT

max

mainsRMS

)

⋅

OUT

max

mainsRMS

)

⋅

−

⋅

() ()L

max

OUT

mainsRMS

OUT

mainsRMS

OUT

⋅

−

⋅

()

max

mains

OUT

mains

OUT

⋅

−

⋅

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