iWatt brings back opto-isolators to tackle output droop
Primary side sensing on dc-dc converters has the great advantage that it eliminates the need for feedback components, particularly opto-isolators, in low-power mains power supplies – for phone charging and LED driving for example.
Algorithms and topologies have developed to the point that 5% output current and voltage regulation is possible.
However, amongst the disadvantages of primary-side sensing is that sensing, and therefore regulation, is only possible when the conversion transformer is being driven.
Heavy instantaneous loads applied between primary power pulses have only the output capacitor to supply them, and this energy will not be replaced until the next power pulse. So output dips are inevitable, particularly if burst-mode is being used – and burst-mode is the default route to minimise no-load power in this kind of chip, with particularly long gaps between bursts.
Aware that large output capacitors are unpopular for size and cost reasons, power chip makers are beginning to offer part-time secondary-side sensing that only operates during output power dips.
In the last few weeks NXP introduced the TEA1705, intended to be paired with the TEA1720A 12W primary side controller, that delivers a signal back across the isolation transformer to kick the primary side into operation if a dip occurs.
This allows the pair to implement a converter with 10mW stand-by and good sudden load response.
To improve output transient response, either can work with a new partner chip: the iW628 voltage monitor.
The monitor sits on the secondary side and signals to the primary side when an output dip occurs.
Like the NXP device, the secondary-side chip sits in low-power mode until a dip occurs, and is therefore close to negligible in power loss.
Unlike NXP, iWatt has chosen to use an opto-isolator as a feedback mechanism – ironically a component that all primary-side controller firms, including iWatt, have decried as unreliable when advocating their primary-side sensing techniques.
However, iWatt claims its technique is the fastest, giving “designers the option for what the company believes is the industry’s fastest dynamic load response”, it said.
Unfortunately for comparison, neither NXP or iWatt is making proper data sheets publically available for any of these components, only product briefs.
According to iWatt, the efficiency of its new devices exceeds proposed 2012 US Department of Energy regulations for ac/dc adapter standby power consumption and efficiency.
They include user-configurable, five-level cable drop compensation to allow thin leads to be used, and adaptive multi-mode PWM/PFM optimises efficiency, EMI, and power consumption.