Resonant Power Supply Suits Audio Systems

Jun 1, 2008 12:00 PM
By Dr. Mahesh de Silva, Senior Applications Engineer, CamSemi, Cambridge, U.K.

A power supply with a novel resonant topology enables an audio system with the efficiency, no-load and size advantages of SMPS, but very low-generated EMI.

Audio equipment manufacturers are under growing market and commercial pressures to improve the efficiency and no-load power dissipation in products that currently rely on linear power supplies. Linear power supplies are ideal for audio applications, because the mains frequency transformer generates low electromagnetic emissions, and because of the minimal design time typically required for low-cost applications with relaxed voltage regulation, ripple and protection specifications. However, linear supplies suffer from low average efficiency and high no-load power, so they struggle to meet today's main regulatory requirements, such as ENERGY STAR and California Energy Commission (CEC).

A typical 12-W linear power supply, for example, has an average efficiency of about 63%, whereas the proposed ENERGY STAR V2.0 requirement, scheduled for implementation later this year, is 77.8%. The no-load power dissipation of around 1.5 W also fails to meet the 300 mW demanded by the standard. Bulky linear supplies, particularly at higher powers, are also getting increasingly expensive because of steep rises in global commodity prices, such as copper and steel used in the line-frequency transformer.

Although low electromagnetic interference (EMI) is often cited as the single most attractive feature of linear supplies for the audio market, the presence of twice the mains frequency ac component at the rectified output can cause audible hum in some applications. This usually gets worse with increasing load and reduced input voltage, which causes the audio quality to deteriorate.

SMPS for Audio Applications

To overcome these efficiency and EMI difficulties, audio power-supply manufacturers are actively looking to replace linear supplies. Specifically, they are focusing increased attention on common switch-mode power-supply (SMPS) topologies such as flyback and ringing choke converter. Both topologies offer higher efficiency, lower standby power and additional features such as overvoltage, overcurrent and overtemperature protection. SMPS also provides tight load and line regulation, which relaxes the requirement for post-regulation circuitry. And with tightly controlled output V-I characteristics, these alternative approaches can be programmed to deliver the peak load capability that many audio systems demand.

On the other hand, SMPS suffers from higher bill-of-materials (BOM) cost and longer design times, making them a much less attractive option as linear replacements in low-cost, high-volume applications. The presence of excessive electromagnetic noise due to fast switching transients is also a major hurdle, because the resulting conducted and radiated emissions interfere considerably with the audio signal. To overcome this, expensive electromagnetic compatibility (EMC) suppression filters are normally required along with EMI reduction techniques in the core of the SMPS controller.

One SMPS design technique involves using a sync pulse from the audio system to dynamically shift the operating frequency away from the instantaneous radio frequency, thereby reducing interference. Switching-frequency dithering or spread-spectrum modulation is another commonly employed method to spread the spectral energy of noise while maintaining the system's overall efficiency. However, even with extensive filtering and sophisticated control techniques, it can be extremely difficult to achieve the very low EMI required by most audio systems to deliver targeted signal-to-noise ratios.

Resonant Topology Replacement

Resonant topologies offer a commercially viable power-supply alternative to overcome the limitations of linear and common SMPS topologies, while meeting the latest efficiency, no-load power and EMI requirements. By switching at near-zero voltage and current, this approach minimizes switching losses to deliver high efficiency and generate minimal EMI because of their sinusoidal switching waveforms. But until recently, resonant topologies have not been exploited commercially for low-power applications in the consumer electronics market because of inherent difficulties in control and the resulting high BOM cost.

A novel single-switch resonant discontinuous forward-converter (RDFC) topology offers the efficiency, no-load and size advantages of SMPS without the cost penalty, as well as additional safety and protection features. More importantly for audio and other EMI-sensitive applications, such as cordless-phone adapters and modem/router power supplies, the topology offers resonant power conversion with naturally low EMI.

As no energy is stored within the forward-mode transformer during switching, a forward-converter topology also allows a reduction in the transformer core size. This delivers a cost benefit in itself, while removing the need for a secondary freewheeling diode and choke to make the solution much more commercially attractive at low power.

Fig. 1 shows the key components of the RDFC topology. The input capacitor (C_IN) smooths the rectified ac voltage at the input and applies it to the forward-mode transformer. Closing the primary switch transfers power from the primary to the secondary during the same conduction phase. The current waveform through the primary transistor consists of current through the leakage inductance and magnetizing inductance. The leakage current component usually dominates and also appears across the secondary diode.

When the primary switch is closed, the total current through the transformer diverts to the resonant capacitance (C_RES), which includes the transformer winding capacitance and primary transistor output capacitance. C_RES forms a resonant circuit with the transformer leakage inductance (L_LEAK) followed by the magnetizing inductance (L_MAG).

Resonant frequencies are given by Eqs. 1 and 2:

where f_RES1 equals the resonant frequency due to the transformer leakage inductance, f_RES2 equals the resonant frequency due to the transformer magnetizing inductance, L_LEAK equals the transformer leakage inductance, L_MAG equals the transformer magnetizing inductance and C_RES equals the transformer resonant capacitance.

The leakage inductance is much smaller than the magnetizing inductance, so the resonant frequency in Eq. 1 is higher than that of Eq. 2.

A mixed-signal control IC developed by CamSemi ensures the RDFC circuitry operates at optimum performance levels with load variations. The resultant C2470 family of controllers achieves this through three main control mechanisms:

• Resonant control senses the resonance waveform to identify the near-zero turn-on and turn-off voltages and to determine the optimum on-time in the next switching cycle.

• Power control is achieved by sensing the switch current and limiting it under overload conditions or reducing the on-time at low-load conditions to minimize no-load power loss.

• Base-drive control dynamically maintains the on-state voltage of the power transistor at an optimum voltage to reduce conduction losses and minimize turn-off time to lower switching losses.

The RDFC controller uses a combination of these three control mechanisms to define five main operating modes of the power supply (Fig. 2):

• Normal mode provides fully resonant switching and has a fixed duty cycle for power delivery from around 20% to 100% load.

• Standby mode is entered as the load decreases. The controller enters this mode by reducing the on-time and increasing the off-time.

• Overload mode occurs under high-output loads. It limits the peak switch current and reduces the on-time, while maintaining fully resonant operation.

• Foldback mode occurs under excessive output loads and reduces the on-time to a minimum, while increasing the off-time to protect the power supply in short-circuit conditions.

• Power-burst mode has an increased duty cycle. The controller enters this mode periodically during the foldback mode to allow the power supply to recover from a short-circuit condition.