How to achieve the lowest possible standby power consumption?

    The latest energy-saving programs require standby power consumption to be less than 75mW and 100mW. Some industry-leading programs are rewarding products that keep standby power consumption below 30mW. Where do we declare victory in the pursuit of no backup power?

    The industry has established that anything below 5mW is considered “zero power” standby loss. This definition is provided by the International Electrotechnical Commission (IEC) 62301, clause 4.5.

    Operating at very low switching frequencies is crucial for achieving the lowest possible standby power consumption. Between switching pulses, the AC/DC controller goes to sleep. At each switching pulse, the controller wakes up long enough to monitor the load conditions and determine how often it should continue operating.

    What happens if a load step occurs while the controller is asleep? Obviously, the output voltage will drop until the next switching event, which can be a very long time. This limits the minimum switching frequency of the controller in standby mode to around 200Hz without requiring a very large output capacitor. At this frequency, standby power consumption levels can easily be kept below 20mW.

    To achieve standby power consumption levels below 5mW, a minimum switching frequency of tens of Hertz is required. By using a simple wake-up monitor, the main controller can operate at these incredibly low switching frequencies without sacrificing load step response. Figure 1 shows a simplified schematic.

    Figure 1: A wake-up monitor enables very low switching frequencies, minimizing standby power consumption.

    The wake-up monitor is located on the secondary side of the transformer and continuously monitors the output. Once a load is detected, the wake-up IC pulls the transformer secondary winding low for a short period of time, telling the primary-side controller to wake up immediately and respond to the increased load demand.

    The UCC28730 is an isolated kickback power supply controller that uses primary-side winding sensing to provide accurate voltage and constant current regulation, eliminating the need for an optocoupler feedback circuit. The controller operates in discontinuous conduction mode with valley switching to minimize switching losses. The modulation scheme combines frequency modulation with primary peak current modulation to provide high conversion efficiency across the load range. The control law provides a wide output power dynamic operating range, facilitating the achievement of <5mW standby power.

    At low power operating levels, the device includes power management features to reduce device operating current at switching frequencies below 28 kHz. The UCC28730 includes pulse width modulator features to reduce peak electromagnetic interference energy at the fundamental switching frequency and its harmonics. Primary-side control enables accurate voltage and current regulation, fast dynamic response, and fault protection. A complete charger solution can be implemented with a simple design process, low cost, and low component count.

    The UCC24650 is an easy-to-use secondary-side voltage monitor that periodically measures its own VDD voltage. When it fluctuates by 3% relative to the previous reading, it sends an alarm signal to the receiving primary-side regulation (PSR) controller. The device's low power consumption helps achieve zero-power standby losses of less than 5mW in many applications.

    An internal 200V MOSFET switch is connected to the ground-referenced rectifier to provide current-limited pulses to the flyback converter's secondary winding, coupling a signal to the primary-side controller. This signal is repeated at 33kHz until controller-driven switching activity is detected.

    The relative float detection function allows voltage regulation to any voltage between 5V and 28V. The wake-up alarm function enables ultra-low standby frequency, minimizing switching losses and reducing the output capacitance required to respond to heavy load steps. This monitor is designed for use with controllers capable of detecting wake-up signals, such as the UCC28730 PSR flyback controller. The UCC24650 also provides enable and disable signals for controlling secondary-side circuitry under no-load conditions, thereby reducing standby power consumption. Such circuits can include synchronous rectifier controllers or relay drivers.

    Features

    ■Excellent load transient performance and zero standby power consumption

    ■Achieve minimum output capacitance to reduce ΔV-OUT

    ■No external components required

    ■<50μA device current compensation (typical)

    ■5V to 28V output monitoring capability

    ■3% voltage floating detection (patent pending)

    ■200V wake-up switch

    ■Can enable and disable SR controller, relay control or other auxiliary circuits

    ■5-pin small outline transistor (SOT)-23 package