Taming the Laser Beam: Synchronized DC/DC Solutions for Low-Noise Lasers
This article was originally published on: eetimes.com/powering-high-precision-lasers/
Contents
- The Evolution and Power Challenges of High-Precision Laser Systems
- From Flash Lamps to Quantum Diode Lasers
- Modern Redesign Requirements: Scalability & Low-Noise Demands
- Understanding Noise and Interference in Multi-Converter Systems
- High-Frequency Noise and Operating Bandwidth
- The "Beating Frequencies" Phenomenon in Switched-Mode Power Supplies
- Limitations of Traditional LC Filtering
- Synchronization to prevent interference
- External Clock Synchronization for Noise Band Management
- Complete Power Solution for High-Precision Laser Drivers
- Summary: Best Practices for Low-Noise Laser Power Design
- Next Steps: Optimize Your Laser Power Architecture
The Evolution and Power Challenges of High-Precision Laser Systems
From Flash Lamps to Quantum Diode Lasers
Today, no one can imagine life without the vast number of laser applications (light amplification by stimulated emission of radiation), ranging from milliwatt-class lasers for scanners, measuring devices, and data transmission to lasers with power levels of several kilowatts for material processing and military applications.
It took many years from Max Planck’s quantum theory proposed in 1900 and Einstein’s theory of stimulated emission of radiation described in 1917 to the first working laser presented in 1960 by Theodore Maimann in California. These first lasers were bulky and used ruby crystals and flash lamps to emit a concentrated beam of coherent red light.
Within a few years, more powerful lasers based on different materials including gases were developed. The concept of a GaAsP (gallium arsenide phosphide) laser diode was published as early as 1962, and significantly more efficient diode lasers were developed in the early 1970s.
Basic research on laser technology is complex, so it is not surprising that several Nobel laureates have been involved in it. Even today, high-tech applications require in-depth knowledge of laser technologies, photonics and optics — a level of expertise only available at a limited number of specialized companies.
Modern Redesign Requirements: Scalability & Low-Noise Demands
A customer producing diode laser systems for quantum science and technology was looking for a power supply solution not interfering with noise sensitive signals in the application. The legacy system used a customized AC/DC power supply with a 15V/100Wpeak output plus two low power outputs providing 5V for a microcontroller board and the additional -15V for Op-Amps. For the redesign the customer was looking for scalable solutions providing up to 200Wpeak and working from a standard industrial 24Vdc bus for easier deployment in scientific and industrial environments. Depending on the configuration multiple converters can be used in one system.
Understanding Noise and Interference in Multi-Converter Systems
High-Frequency Noise and Operating Bandwidth
Interference:
The topic of interference between two waves of different frequencies is quite complex, and the result also depends on how much the frequencies differ. In this article, therefore, we will focus only on the effects as they relate to power supplies.
Due to switching high voltages and currents at frequencies of several hundred kilo Hertz every power converter is a source of high frequency noise. Appropriate EMI filters prevent negative effects on a system. However, if there are other sources of high-frequency noise in a sensitive system, unwanted interference may occur. For this reason, developers of highly sensitive systems ensure that potential interference frequencies do not fall within the operating bandwidth of critical signals.
The complexity is demonstrated by the interference waveform in figure 1, generated by the superposition of a 250kHz and a 400kHz sine wave.
Figure 1: Interference of a 250kHz and a 400kHz sine wave
The "Beating Frequencies" Phenomenon in Switched-Mode Power Supplies
This applies not only to interference between a switched-mode converter and other high-frequency signals in the system, but also to interference between multiple converters. Frequencies of switched-mode power supplies usually have a tolerance of around ±10%. If the frequencies of two superimposed waves are close together, the result of the interference is different to the graph in figure 1. This phenomenon is known in the power industry and you may find the term “beating frequencies” in the literature.
If, for example, two converters are used in a system, one switching at 250kHz and the other at 255kHz, an interference waveform with a frequency of 
is generated. The amplitude is the sum of both waves and varies with a 5kHz envelope (famplitude = f1 + f2). This effect is shown in figure 2.
Figure 2: Interference of 2 sine waves with slightly different frequencies
Adding to this system another converter with a switching frequency of, for example, 245 kHz would generate an even more complex mix of interferences that could disrupt a sensitive system.
Limitations of Traditional LC Filtering
Some suppliers of switched-mode power converters suggest additional inductance shown in figure 3 to reduce this interference. But it increases losses and larger capacitors are needed to keep impedances low.
Figure 3: Additional inductances reduce interference between different switching frequencies
Synchronization to prevent interference
P-DUKE’s extensive product range of DC/DC converters also includes several converters with synchronisable switching frequencies. If all converters in a system are working with the same frequency, interferences with other frequencies are significantly reduced and limited to a well-defined bandwidth.
For this application P-DUKE suggested the HAE200-24S15-YC, a 200W module with 24Vnom input, 15Vout and the Sync option. In the basic configuration shown in figure 4 the Sync pins of two modules are connected, and both modules work with the same switching frequency.
Figure 4: Switching frequencies synchronized by using the Sync pin
Up to 3 modules can be synchronized by this technology and the module with the highest switching frequency defines the frequency of the array.
External Clock Synchronization for Noise Band Management
But these modules offer even more flexibility as modules in a system can also be synchronized by an external clock with a maximum voltage of 5V referenced to the – input pin. The clock frequency must be higher than the highest regular switching frequencies of modules in the array and must be kept within the limits given by the magnetic design of the converter. For more details, please contact P-DUKE’s application engineering.
Figure 5: Synchronization by an external clock signal
This allows a customer to not only have all modules working with the same switching frequency but also move the frequency of a single, dual or triple module configuration away from any noise-sensitive band within the application.
Complete Power Solution for High-Precision Laser Drivers
The 5V/2A and -15V/500mA supply voltages needed in the system are generated from the 15V bus by additional converters from P-DUKE. Due to the low power, they don’t produce any critical interference with other components in the system.
The PSR02-12S05 is a non-isolated converter with the industry standard LM78 pinout and can provide 5V/2A from input voltages ranging from 6.5 to 36V
The UDH01-12S15 is an isolated 1W converter with a 15V/1W output, enough power to provide the negative supply voltage for the OP-Amps in high precision circuits. By connecting the positive output to system ground, a -15Vdc supply voltage is generated.
Figure 6 shows the complete block diagram of the laser driver system.
Figure 6: Power supply for the laser system
Summary: Best Practices for Low-Noise Laser Power Design
Interference in a system is quite often difficult to detect but can lead to malfunctions or even failures in sensitive circuits. Understanding the basics is important to prevent interference and to analyze malfunctions in a system that seem inexplicable at first glance.
With a broad portfolio of low-noise converters and world-class technical support, P-DUKE can assist customers in selecting the right components and, later on, in troubleshooting. Please contact P-DUKE’s application engineering in case of any questions.
Next Steps: Optimize Your Laser Power Architecture
Mitigating beating frequencies and switching noise requires a synchronized power strategy to protect sensitive signals.
- Explore Solutions: Review technical specifications for the HAE200, PSR02, and UDH01 series.
- Request Samples: Contact our sales team to evaluate synchronized DC/DC modules on your test bench.
- Consult FAE: Discuss external clock synchronization, peripheral rail decoupling, or noise troubleshooting with our Application Engineering Team.
