NCP1015ST65T3G datasheet(10/22 Pages) ONSEMI

Part #	NCP1015ST65T3G

Description	Self-Supplied Monolithic Switcher for Low Standby-Power Offline SMPS

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10

Plugging Equation 7 and Equation 8 into Equation 6 leads

to <Vds(t)> = Vin and thus:

PDSS + Vin @ ICC1

(eq. 9)

The worse case occurs at high line, when Vin equals

370 Vdc. With ICC1 = 1.2 mA (65 kHz version), we can

expect a DSS dissipation around 440 mW. If you select a

higher switching frequency version, the ICC1 increases and

it is likely that the DSS consumption exceeds 500 mW. In

that case, we recommend adding an auxiliary winding in

order to offer more dissipation room to the power MOSFET.

Please read application note AND8125/D “Evaluating the

power capability of the NCP101X members” to help

selecting the right part / configuration for your application.

Lowering the Standby Power with an Auxiliary

Winding

The DSS operation can bother the designer when a) its

dissipation is too high b) extremely low standby power is a

must. In both cases, one can connect an auxiliary winding to

disable the self−supply. The current source then ensures the

startup sequence only and stays in the off state as long as

VCC does not drop below VCC(on) or 7.5 V. Figure 17 shows

that the insertion of a resistor (Rlimit) between the auxiliary

dc level and the VCC pin is mandatory a) not to damage the

internal 8.7 V zener diode during an overshoot for instance

(absolute maximum current is 15 mA) b) to implement the

fail−safe optocoupler protection as offered by the active

clamp. Please note that there cannot be bad interaction

between the clamping voltage of the internal zener and

VCC(off) since this clamping voltage is actually built on top

of VCC(off) with a fixed amount of offset (200 mV typical).

Self−supplying controllers in extremely low standby

applications often puzzles the designer. Actually, if a SMPS

operated at nominal load can deliver an auxiliary voltage of

an arbitrary 16 V (Vnom), this voltage can drop to below

10 V (Vstby) when entering standby. This is because the

recurrence of the switching pulses expands so much that the

low frequency re−fueling rate of the VCC capacitor is not

enough to keep a proper auxiliary voltage. Figure 18 shows

a typical scope shot of a SMPS entering deep standby

(output un−loaded). So care must be taken when calculating

Rlimit 1) to not excess the maximum pin current in normal

operation but 2) not to drop too much voltage over Rlimit

when entering standby. Otherwise the DSS could reactivate

and the standby performance would degrade. We are thus

able to bound Rlimit between two equations:

Vnom * Vclamp

Itrip

v Rlim v

Vstby * VCC(on)

ICC1

(eq. 10)

Where:

Vnom is the auxiliary voltage at nominal load

Vstdby is the auxiliary voltage when standby is entered

Itrip is the current corresponding to the nominal operation.

It thus must be selected to avoid false tripping in overshoot

conditions.

ICC1 is the controller consumption. This number slightly

decreases compared to ICC1 from the spec since the part in

standby does almost not switch.

VCC(on) is the level above which Vaux must be maintained

to keep the DSS in the OFF mode. It is good to shoot around

8 V in order to offer an adequate design margin, e.g. to not

re−activate the startup source (which is not a problem in

itself if low standby power does not matter)

Since Rlimit shall not bother the controller in standby, e.g.

keep Vaux to around 8 V (as selected above), we purposely

select a Vnom well above this value. As explained before,

experience shows that a 40% decrease can be seen on

auxiliary windings from nominal operation down to standby

mode. Let’s select a nominal auxiliary winding of 20 V to

offer sufficient margin regarding 8 V when in standby (Rlimit

also drops voltage in standby). Plugging the values in

Equation 10 gives the limits within which Rlimit shall be

selected:

20 * 8.7

6.3 m v

Rlimit v

12 * 8

1.1 m

(eq. 11)

that is to say: 1.8 k

W < Rlimit < 3.6 kW.

If we are designing a power supply delivering 12 V, then

the ratio auxiliary/power must be: 12 / 20 = 0.6. The ICC

current has to not exceed 6.4 mA. This will occur when Vaux

grows−up to: 8.7 V + 1.8 k x (6.4 m + 1.1 m) = 22.2 V for

the first boundary or 8.7 V + 3.6 k x (6.4 m +1.1 m) = 35.7 V

for second boundary. On the power output, it will

respectively give 22.6 x 0.6 = 13.3 V and 35.7 x 0.6 = 21.4 V.

As one can see, tweaking the Rlimit value will allow the

selection of a given overvoltage output level. Theoretically

predicting the auxiliary drop from nominal to standby is an

almost impossible exercise since many parameters are

involved, including the converter time constants. Fine

tuning of Rlimit thus requires a few iterations and

experiments on a breadboard to check Vaux variations but

also output voltage excursion in fault. Once properly

adjusted, the fail−safe protection will preclude any lethal

voltage runaways in case a problem would occur in the

feedback loop.

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