Switch mode VS Phase Controlled

Switch mode VS Phase Controlled

Magellan Power Comparison 
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Which is Best?
By Masoud Abshar Managing Director Magellan Power.

During my 35 years’ experience as a Power Electronic Engineer and 28 years as founder and Managing Director of Magellan Power, people have repeatedly asked me to comment on the comparative merits of both Switch Mode and Phase Controlled technology. At Magellan Power we manufacture both Switch Mode and Phase Controlled equipment so this is my view of the two technologies. 

To answer these questions it is helpful to do a quick review of the two technologies. The two diagrams below clarify the discussion:

Diagram 1: Switch Mode Technology

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With reference to Diagram 1: 

A typical switch-mode rectifier consists of: 

Front end rectifier: This converts the mains typically from 240V to 340VDC (max). 

Power factor correction stage: This is essentially a DC-DC converter which converts 340V to approx 450VDC. 

PWM stage: This chops the 450VDC at high frequency (typically 25-40kHz) and alters the mark/space ratio. This is typically performed by a transistor.

High frequency transformer: To isolate the input mains to low voltage output, a small, high frequency ferrite transformer is used. This is the main cost saving compared to a phase controlled rectifier which uses a 50Hz transformer.

Output rectification: The high frequency pulses out of the transformer are rectified and the resultant DC is further smoothed using electrolytic capacitors.

Total conversion stages: 5  

Diagram 2: Phase Controlled Technology  
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With reference to Diagram 2: 

A typical phase controlled rectifier consists of:

Input Transformer: This steps down the input mains to a voltage close to the required DC output. Say for a typical 48V DC system, the transformer output is 70VAC.

Thyristor Bridge: This converts the transformer output into a controlled DC voltage.

Total conversion stages: 2

The main advantage switchmode rectifiers have is that they are smaller and lighter (mainly due to high frequency rectification) and cost less than their phase controlled counterparts. They also have hot swappable capability. 

Pros and Cons of Both: 

Size and Weight: 
• Switch mode is smaller and lighter. However this may not be an advantage when it is combined with a battery bank as the battery is the heavy and bulky component in a typical substation application.

Reliability/Durability: 
• Phase controlled is much more reliable. It has two power conversions compared to five for switch-mode. In its basic form a phase controlled rectifier can work reliably for 40 years. It can operate in a dusty environment at a much higher temperature and humidity.
• Due to its input transformer, a phase controlled DC system has a much higher tolerance of  electrical noise and input voltage transients/spikes. 
• A thyristor used in a phase controlled rectifier is physically larger and more rugged than a transistor used in a switch-mode rectifier.

Input/Output Power quality:
• Switch-mode technology has a much higher input power factor than phase controlled technology.
• Both switch-mode and phase controlled rectifiers can have low output ripple content.  

Functionality and Features:
• Magellan phase controlled MCRII charger/rectifiers have exactly the same functionality and features as Magellan switch-mode charger/rectifiers as these functions are supplied by the controller which is common between the two technologies.

Service life:
• Phase controlled technology has a much higher service life. In a typical substation application a phase controlled rectifier/charger can last 25-30 years.

In summary - how to select a charger:

• For reliability and longevity, phase controlled technology is a better choice.
• For a hot, dusty and humid environment, phase controlled is a better choice.
• If you require a smaller size and weight of rectifier, then switch mode is a better option.
• For restricted budgets, switch mode can be cheaper.
• If input power factor is important then switch mode is a better choice.

For more information please see our phase controlled series MCRII, or our switch mode range SMCRII - both designed and manufactured in Australia. 
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