On-Board Power Supplies for Optic Modules
Mar 8, 2024
An optic module is a transceiver device that converts high speed electrical data signals into optical signals and vice versa. Such electro-optic systems are key components in high-speed communication systems, enabling the transmission of data over very long distances of up to tens of kilometers with minimal signal loss or alternatively over much shorter distances of a few meters but at very high data rates of hundreds of gigabytes per second, where the losses and delays due to the capacitance and inductance of electrical data connector cables would be unacceptable. These optical communication systems are used in many applications, including telecommunications, HPC data centers, and high-speed networking equipment.
Fig. 1: Operating principle of a fiber-optic communications network
Optic modules consist of a laser diode optical transmitter system called the TOSA (Transmitting Optical System Assembly) and a high-speed photodiode receiver called the ROSA (Receiving Optical System Assembly).
The TOSA converts electrical signals into optical signals for transmission over the optical fiber, while the ROSA converts incoming optical signals back into electrical signals. Full duplex communication is possible when connected with two fiber-optic connectors (Figure 1).
The TOSA converts electrical signals into optical signals for transmission over the optical fiber, while the ROSA converts incoming optical signals back into electrical signals. Full duplex communication is possible when connected with two fiber-optic connectors (Figure 1).
Fig. 2: Example of an optical module
Optical transceiver modules come in various standardized form factors, such as Small Form-factor Pluggable (SFP) and Quad Small Form-factor Pluggable (QSFP), and others. These form factors define the physical and electrical characteristics of the module and allow for interchangeability in compatible 19” rack assemblies.
For example, the QSFP module dimensions are set to 8.5mm height, 18.35mm width and 72.4mm length. Squeezed inside this compact module are the fiber-optic connectors, laser diode driver, photodiode amplifier, digital signal processor, data connector and on-board power supplies.
For example, the QSFP module dimensions are set to 8.5mm height, 18.35mm width and 72.4mm length. Squeezed inside this compact module are the fiber-optic connectors, laser diode driver, photodiode amplifier, digital signal processor, data connector and on-board power supplies.
Fig. 3: Example of an optical module power tree
As the desired data rates increase from tens of Gbps to hundreds of Gbps, the optical performance of the laser diode and photodiode become critical limiting elements, so the ROSA and TOSA need to have adjustable biasing power supplies to accelerate the reaction time to 50ps or less, further complicating the power supply system.
Furthermore, the optical properties of the laser diode are strongly temperature dependent – if the temperature changes by more than ±0.1°C, the wavelength of the light output will drift, potentially harming the performance of the laser diode. One way of maintaining a constant laser diode temperature is to use an active Peltier element for thermoelectric cooling (TEC).
The power tree of the internal power supply becomes complex, requiring several DC/DC converter devices to provide the various on-board voltages needed (Figure 3).
Furthermore, the optical properties of the laser diode are strongly temperature dependent – if the temperature changes by more than ±0.1°C, the wavelength of the light output will drift, potentially harming the performance of the laser diode. One way of maintaining a constant laser diode temperature is to use an active Peltier element for thermoelectric cooling (TEC).
The power tree of the internal power supply becomes complex, requiring several DC/DC converter devices to provide the various on-board voltages needed (Figure 3).
Fig. 4: Optical module power tree with sockets and dimensions
DC/DC converter modules rather than low drop out (LDO) linear regulators are needed because the efficiency requirements are high and self-heating due to internal power losses must be kept to an absolute minimum. The compact optical module size format also means that the height of the DC/DC converters must be kept the same as, or smaller than the other ICs on the board.
In practice, this means a converter height limitation of 2mm or less. And, of course, the footprint of the converters must also be as small as possible as the main PCB is restricted in width and length. A final limitation is the supply voltage of only 3.3V DC. Most compact DC/DC converter modules with integrated inductors and high output currents are designed to operate from a higher supply voltage of 5V or 12V DC (4-16V DC).
To meet all these demanding technical requirements, RECOM has released the RPZ series of sub-miniature DC/DC converter modules. The RPZ-6.0 is an SMD power module in a 4mm x 6mm x 1.6mm QFN package operating from a 2.75 – 7V DC input voltage and delivering a regulated 0.6V - 6.65V DC adjustable output at up to 6 amps with 90% conversion efficiency. The RPZ shielded inductor is integrated, so only external capacitors and set resistors are needed to make a complete power supply.
In practice, this means a converter height limitation of 2mm or less. And, of course, the footprint of the converters must also be as small as possible as the main PCB is restricted in width and length. A final limitation is the supply voltage of only 3.3V DC. Most compact DC/DC converter modules with integrated inductors and high output currents are designed to operate from a higher supply voltage of 5V or 12V DC (4-16V DC).
To meet all these demanding technical requirements, RECOM has released the RPZ series of sub-miniature DC/DC converter modules. The RPZ-6.0 is an SMD power module in a 4mm x 6mm x 1.6mm QFN package operating from a 2.75 – 7V DC input voltage and delivering a regulated 0.6V - 6.65V DC adjustable output at up to 6 amps with 90% conversion efficiency. The RPZ shielded inductor is integrated, so only external capacitors and set resistors are needed to make a complete power supply.
The RPZ-3.0A power module is even smaller with a 2.5 x 3.5mm footprint and 1.6mm height. It can deliver up to 3A with an adjustable output voltage from 0.6 – 5.5V DC. Finally, the RPZ-1.0 can deliver 1A with an adjustable output voltage from 0.6 – 5.25V DC in a tiny 2 x 2mm footprint (the same size as a linear regulator, but with much higher efficiency). All RPZs are short circuit proof.
The completed optical module power tree with suitable sockets is as shown in Figure 4. The PCB needs only a 1.6mm gap to the housing to accommodate the power supply modules.
A summary of all the RPZ series specifications is shown below:
| Series | Input Voltage | Output Voltage | Output Current | Efficiency |
|---|---|---|---|---|
| RPZ-0.5 | 2.3 – 5.5V DC | 0.6 - 5.375V DC | 0.5A | 88% |
| RPZ-1.0 | 2.3 – 5.5V DC | 0.6 - 5.25V DC | 1A | 88% |
| RPZ-2.0 | 2.75 – 6.0V DC | 0.6 – 5.74V DC | 2A | 90% |
| RPZ-3.0A | 2.75 – 6.0V DC | 0.6 – 5.5V DC | 3A | 92% |
| RPZ-6.0 | 2.75 – 7.0V DC | 0.6 – 6.65V DC | 6A | 90% |
All the RPZ power modules use a constant on-time (COT) current control scheme which eliminates the need for a feedback compensation network and enables an ultra-fast load regulation to transients and very low quiescent current. This, combined with short circuit protection, over-current protection, over-temperature protection, and under voltage lockout makes them ideal for powering both digital signal processors and precision analog amplifier loads.
As mentioned previously, the standard supply voltage for optic modules is 3.3V. This low output voltage is rare in high power AC/DC converters, so a high output current DC/DC converter is needed for the primary 3.3V bus supply. RECOM also manufactures the RPMGS5.0-20, a sixteenth brick (34 x 37mm) open frame DC/DC converter that delivers 20A at 3.3V DC from an 18-75V DC supply, allowing industrial 24V, 48V or PoE inputs to be used, or cost effective universal input AC/DC input power supplies can be used, such as the RACM90E-48SK with a built-in Class B EMC filter in a compact 2”x4” footprint.
Applications
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| 1 |
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RACM90-K
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| 2 |
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RPMGS-20
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| 3 |
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RPZ-0.5
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| 4 |
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RPZ-0.5-EVM-1
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| 5 |
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RPZ-1.0
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| 6 |
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RPZ-1.0-EVM-1
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| 7 |
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RPZ-2.0
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| 8 |
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RPZ-2.0-EVM-1
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| 9 |
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RPZ-3.0A
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| 10 |
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RPZ-3.0A-EVM-1
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