Self-Powering the Controller in Standby - Application Note - AND8076/D

Oct 12, 2026

Upload by ahduag

Application note prepared by Christophe Basso for ON Semiconductor explains how to reach ultra-low standby power using the NCP120x-series, focusing on the NCP1200 approach. It compares DSS versus non-DSS operation, detailing overload detection, guaranteed behavior at low output levels, and tradeoffs in standby power and power dissipation. It then describes self-powering the controller in standby with an auxiliary winding, including requirements to keep Vcc above UVLO, prevent re-activation of DSS in no-load conditions, and meet maximum auxiliary Vcc ratings for a 70 W, 16.5 V design target.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages14

Reading Time35 min

AccessFree to download

Introduction

DSS or Not DSS?

Standby power impact

Power dissipation considerations

Self-Powering the Controller in Standby

Auxiliary winding and UVLO requirements

Auxiliary clamping and transformer ratio

AND8076/D A 70 W Low Standby Power Supply with the NCP120x Series Prepared by: Christophe Basso ON Semiconductor APPLICATION NOTE INTRODUCTION The NCP1200 represents one of the cheapest solutions to build efficient and cost-effective Switch-Mode Power Supplies (SMPS). As this design example will show, the part definition does not confine the component in low-power applications only, but it can actually be used in Flyback and Forward supplies for virtually any output power. The below example depicts a universal mains 90-260 VAC power supply delivering 16.5 V @ 4.5 A. Beside its ease of implementation, the NCP1200 excels in true low standby power designs. This application note details how an amazing standby power of less than 100 mW can be reached at high line with a nominal 70 W board. DSS or Not DSS? The Dynamic Self Supply (DSS) lets you directly drive MOSFETs from the high-voltage rail. This option brings you several advantages, as stated below: • True overload detection: with UC384X-based systems, the switching oscillations are stopped in case the Vccline drops below a given Undervoltage Lockout level (UVLO). This principle considers a good coupling between the primary auxiliary winding and the power secondary winding. Unfortunately, leakage elements often degrade this coupling and you only can detect true short-circuit (when Vout is close to zero) and not overload conditions. Thanks to the DSS, the NCP1200 does not need an auxiliary information to sense an overload condition. By detecting a current setpoint pushed to the maximum, the internal logic takes the decision to enter into a safe burst operation, auto-recovering when the default leaves. Precise overload levels can thus be implemented. • Guaranteed operation at low output levels: the Vcc delivered by an auxiliary winding moves with the power output level because a coupling exists between both windings. When the supply is used in battery charging applications, Vout can move depending on the charging state. That is to say, when the battery is nearly empty, its voltage can be close to zero, forcing Vout at this level. Thanks to the natural secondary / auxiliary reflection, the primary auxiliary winding cannot maintain a sufficient voltage on the control IC: Vcc collapses and puts the controller in trouble, probably entering an hiccup mode, similar to that of a startup sequence. DSS being decoupled from Vout, you never see that phenomenon. As you can see, the DSS offers interesting features but, on the other hand, it can sometimes compromise key design parameters. Standby power and power dissipation are one of these: • Standby power: the DSS standby power contribution can easily be evaluated: VHV × Iavg with Iavg, the current consumption taken by the controller and VHV, the high-voltage supply rail. If Iavg equals 1 mA, then we have a standby power of 350 mW at a 350 VDC voltage rail. Tricks exist to slightly reduce it, like the half-wave diode, but you will only gain between 20–30% . • Power dissipation: as stated above, all the current consumed by the IC is seen through pin8 . This is due to the self-adaptive feature of the DSS. Should the IC current move up or down, the DSS duty-cycle will automatically adjust to deliver it. The controller current depends on the internal IC consumption, but also on the type of MOSFET connected to the output. It therefore important to assess the total current drawn from the HV rail and checks the right compatibility with the package type. All details are given in the NCP1200 dedicated data sheet and the application note AND8023/D. As a result, the answer lies behind your design constraints. If you would like to have a precise Over Current Protection (OCP) trip point while driving a moderate size MOSFET, DSS can be a good choice, provided low standby power isnot an absolute necessity. In our case, we want to drive a large MOSFET for a better efficiency but we need to reach the lowest possible standby power

FAQs

What is DSS in the NCP1200 context, and what does it change in SMPS operation?

DSS (Dynamic Self Supply) directly drives MOSFETs from the high-voltage rail, enabling overload-related behavior without relying on auxiliary sensing. It can also ensure operation at very low output levels by keeping DSS decoupled from Vout.

Why can DSS compromise design parameters such as standby power and power dissipation?

Standby power contribution scales with VHV and the controller’s average current consumption, so DSS adds measurable standby power. Power dissipation is also affected because controller current drawn through the DSS duty-cycle reflects the IC behavior and MOSFET-related requirements, making package compatibility important.

How can the controller be self-powered in standby without re-activating DSS?

Permanently disable DSS by guaranteeing an auxiliary level above VccON max (11 V for the NCP1200). If this condition is not met, DSS re-activates under no-load conditions and degrades standby power; additionally, keep auxiliary Vcc below the maximum rating (less than 16 V) using an appropriate winding ratio and clamping (e.g., Zener diode).

Oct 12, 2026

Upload by ahduag