NCP1011AP130 datasheet(11/24 Pages) ONSEMI

Part #	NCP1011AP130

Description	Self?뭆upplied Monolithic Switcher for Low Standby?뭁ower Offline SMPS

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Manufacturer	ONSEMI [ON Semiconductor]

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NCP1010, NCP1011, NCP1012, NCP1013, NCP1014

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11

Plugging Equations 7 and 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 = 1.1 mA (65 kHz version), we can

expect a DSS dissipation around 407 mW. If you select a

higher switching frequency version, the ICC1 increases and

it is likely that the DSS consumption exceeds that number.

In that case, we recommend to add 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 in

selecting the right part/configuration for your application.

Lowering the Standby Power with an Auxiliary Winding

The DSS operation can bother the designer when its

dissipation is too high and 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 VCCON or 7.5 V. Figure 18 shows

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

DC level and the VCC pin is mandatory to not damage the

internal 8.7 V active Zener diode during an overshoot for

instance (absolute maximum current is 15 mA) and to

implement the fail--safe optocoupler protectionas offeredby

the active clamp. Please note that there cannot be bad

interaction between the clamping voltage of the internal

Zener and VCCOFF since this clamping voltage is actually

built on top of VCCOFF 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 refueling rate of the VCC capacitor is not

enough to keep a constant auxiliary voltage. Figure 19

portrays a typical scope shot of a SMPS entering deep

standby (output unloaded). So care must be taken when

calculating Rlimit 1) to not trigger the VCC over current

latch [by injecting 6.3 mA (min. value) into the active

clamp] 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

≤ Rlimit ≤

Vstby − VCCON

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 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 almost does not switch.

VCCON is the level above which Vaux must be maintained

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

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

reactivate 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 8.0 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 8.0 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 ≤

Rlimit ≤ 12 − 8

1.1 m

(eq. 11)

1.8 k < Rlimit < 3.6 k

,thatis tosay:

If we design a power supply delivering 12 V, then the ratio

between auxiliary and power must be: 12/20 = 0.6. The OVP

latch will activate when the clamp current exceeds 6.3 mA.

This will occur when Vaux increases to: 8.7 V + 1.8 k x

(6.4m + 1.1m) = 22.2 V for the first boundary or 8.7 V +

3.6 k x (6.4m +1.1m) = 35.7 V for second boundary. On the

power output, it will respectively give 22.2 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.

When an OVP occurs, all switching pulses are

permanently disabled, the output voltage thus drops to zero.

The VCC cycles up and down between 8.5–4.7 V and stays

in this state until the user unplugs the power supply and

forces VCC to drop below 3.0 V (VCCreset). Below this

value, the internal OVP latch is reset and when the high

voltage is reapplied, a new startup sequence can take place

in an attempt to restart the converter.

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