HV37-08F Fast Recoveryhigh voltage diode 8KV, 400mA, 75nS

Product Overview

• The HV37-08F is a fast recovery high voltage diode and the 8 kV step in the HV37 family. It blocks 8 kV of repetitive peak reverse voltage, carries 400 mA of average forward current in still air at 40 °C, and comes out of conduction in 75 ns — a combination built for high-frequency high-voltage rectification, where the recovery loss of a slower part would use up most of the margin the design was built around.
• Physically it is an axial lead high voltage diode: a black moulded cylindrical body with a white band marking the cathode, and two straight leads. That form goes into a through-hole footprint, a socket or a point-to-point high-voltage assembly, and the leads can be formed so the body lies flat where height is limited.
• Where the model sits is simple. A design that needs more blocking margin than a 6 kV part can give steps up to this one and keeps most of its current; a design already running at 10 kV or 12 kV steps down to it and gains current headroom in the same package format. That is the point of an 8 kV, 400 mA rating — it sits in the middle of the ladder, for supplies that are neither voltage-starved nor current-starved.
• It belongs on the high-voltage side of a supply: the secondary rectifier after a step-up transformer, a multiplier or stack string, a discharge path, a clamp across an inductive high-voltage node. It is not a mains rectifier, and it is not a replacement for a purpose-built high-voltage assembly — it is the discrete part inside one.
• Every rating is tied to a stated test condition. The blocking voltage is quoted at 25 °C against a leakage criterion, both current figures are quoted at their own ambient temperatures, and operation and storage cover the same temperature span. Those conditions are what you compare when other high voltage diode manufacturers quote a headline that looks better on paper; the numbers only mean something next to the conditions they were measured under.
• Axial-leaded connection: two straight leads suit through-hole, socketed and point-to-point high-voltage assemblies, and they can be formed so the body lies flat where vertical space is tight.
• High thermal conductivity epoxy molding: the compound gives heat a short path from the chip into the body and out through the leads, which is how a package this small sustains a several-hundred-milliamp forward current.
• Fast reverse recovery: the short recovery time keeps switching loss low in high-frequency rectification, where a standard-recovery part would dissipate more than it delivers to the load.
• Passivation and chip protection: the chip surface is passivated and sealed, which is what holds the blocking voltage through thermal cycling — the usual failure mode on an unprotected high-voltage chip is surface leakage that creeps up with age and heat.
• Discharge behaviour: the part is built for repetitive discharge and surge events without a lasting shift in blocking voltage, so a supply that flashes over occasionally does not automatically take the diode with it.
• Series consistency: the HV37 range is built on one package format, so high voltage diodes can move up or down the voltage ladder without a board redesign — check the package code for the exact model before you commit tooling.
• Straightforward incoming inspection: the cathode band and the printed model code are visible on the body, so a fast recovery diode can be checked against the drawing without powering the assembly.

Package and PCB Fit

At 8 kV the mechanical drawing is part of the electrical design, not paperwork. The specified body and lead dimensions set the envelope, and general tolerances are ±0.2 mm with a wider ±0.5 allowance on lead length, so a formed lead can end up slightly longer or shorter without forcing a layout change.

The body length is doing real work. It sets the distance between the two points where the leads leave the moulding, which is the surface path the voltage has to cross when the part runs in air. Shorten that body and you shorten that path, which is why a part of this voltage class cannot simply be scaled down.

The lead length gives you room to reach through a board, form a foot for surface mounting, or trim after soldering. The drawing shows both straight leads and formed leads bent for PCB mounting, with the joint covered by insulating adhesive — the arrangement most assemblies use for an axial lead high voltage diode.

Keep the body clear of metalwork, screw heads and neighbouring traces, and treat the coating as part of the insulation system rather than an optional extra. Check the drawing before you finalise a tight footprint: the tolerance on lead length is wider than on the body, and that changes how a formed lead seats in a plated hole.

Operating Environment and Cooling

1. Insulating oil. This is the recommended environment: oil fills the gaps around the body, suppresses corona and carries heat away from the leads, so the in-oil current column is the one that applies. It is the usual choice inside a sealed high-voltage tank or a filled module.
2. Insulating gas. Also recommended. Insulating gases give strong dielectric performance without the weight of oil, but convection is weaker in a gas than in a liquid, so treat cooling as the limiting factor and keep the gas dry and filtered.
3. Secondary packaging. Potting or encapsulating the finished assembly raises dielectric strength and protects the surface, but it also traps heat, so heat dissipation and the remaining parameter margins matter more here than they do in oil or gas.
4. Exposed air or forced air. A small axial lead high voltage diode running in open air discharges more easily at its surface and cools less effectively than the same part in oil. Keep clearances generous, add airflow if the enclosure allows it, and mount high voltage diodes used in exposed air with insulated electrodes at both ends.
5. Whichever route you take, the current column follows the cooling. If the assembly will be potted or run in still air, size the load from the air figure and add your own margin for a hot enclosure rather than assuming the oil figure will hold.

Installation and Mounting

1. Check the footprint before anything is formed. Confirm the hole spacing, the clearance around the body, and that the cathode marking on the board matches the white band on the part.
2. Form the leads with support. Grip the lead between the body and the bend with pliers so the force never reaches the moulding seal, bend once rather than working the lead back and forth, and keep the finished bend clear of the body.
3. Seat the part so the body does not rest on a trace, a screw head or a heatsink edge. At 8 kV the distance from the body and leads to other conductors matters as much as the coating does.
4. Solder with an iron and dwell time that suits the lead, let the joints cool naturally rather than quenching them, and clean flux residue off the body before coating.
5. Apply the insulating adhesive over both lead exits and the body ends, work out air bubbles, and cure it to the schedule for the material you selected.
6. Test before power-up. Run your usual insulation or hipot check on the coated assembly, confirm the orientation of every diode in the string, and make sure no coating has bridged across to a neighbouring node.
7. Record the model and the coating used. When the assembly comes back for service, the same combination is what keeps the replacement identical to the original.

Typical Applications

An 8 kV, 400 mA fast recovery rectifier turns up wherever a supply has to generate a few kilovolts at modest current and switch quickly while doing it. The everyday example is the high-voltage generator in imaging equipment — X-ray tubes, digital radiography, CT and dental panoramic units — where rectification sits after a step-up transformer or inside a multiplier and the diodes have to stay cool in a small enclosure.

Industrial imaging and test equipment uses the same class of part: non-destructive testing sets, cable and insulation testers, high-voltage calibration benches, electrostatic and HVDC supplies, and radar modulator circuits. In each of them the high voltage diode is one link in a chain, and its recovery time and leakage decide how much of the design margin is left for everything else.

Smaller builds put the part in a PCB-mounted multiplier string, where the axial body is formed flat against the board and the whole assembly is then coated or potted for insulation. That arrangement suits this package well, provided the coating is applied without voids and the board keeps its distance from other conductors.

If your rail is lower and your current higher, or your working voltage is well above this step, a different model in the same family fits better without changing the board outline.

Specifications

Reverse Leakage Behavior

Reverse leakage is the current that keeps flowing while the diode is blocking, and it is the parameter that decides whether an 8 kV node holds its voltage once the junction is hot. The HV37-08F is specified at 2.0 µA with the full 8 kV across it at 25 °C, and at 10.0 µA at 100 °C. The hot figure is the one to design with in any circuit that runs warm, because leakage rises steeply with junction temperature and a part that measures clean on a cold bench can still load a high-impedance node after warm-up.

In a high-impedance high-voltage node — a multiplier stage, a bias or clamp feed, a low-current divider — that leakage behaves like a resistor connected across the diode. A few microamps can pull down a node fed through a high-value resistor, shift a threshold, or add an error that only appears after the assembly has reached operating temperature. The effect scales with the impedance of the node, not with the forward current rating of the high voltage diode.

Junction temperature is not ambient temperature. The 100 °C point is a junction condition, and a fast recovery diode dissipating switching and forward loss inside a potted or enclosed assembly can reach it while the outside of the enclosure is much cooler. Use the 100 °C leakage figure for thermal design unless you have measured the junction in your own build. The 25 °C figure is a bench value, useful for incoming inspection and comparison between suppliers, but it is not a safe basis for a hot running supply.

Above 100 °C the datasheet does not publish a further leakage value up to the 125 °C maximum operating ambient. Treat 10.0 µA as the last specified point rather than a ceiling, and leave margin in the node design. If the circuit cannot tolerate the leakage at its real operating temperature, the practical choices are a lower-voltage step with more voltage margin, a different circuit topology, or a measurement on your own assembly at the temperature it actually reaches.

Insulating Adhesive and Surface Coating

• Silicone: cures to a clear, elastic rubber with the highest shock absorption of the four and the widest tolerance of severe temperature swings, staying serviceable from about -40 °C to 200 °C.
• Acrylic: cures to a clear, hard coating with low moisture absorption, a fast cure, good wear resistance and good insulation — a practical choice for a production line that needs a quick, clean finish.
• Urethane: also clear and hard after curing, with excellent wear resistance and good moisture resistance; it stays stable at low temperature but tolerates high temperature less well than silicone.
• Epoxy: very sturdy and usually opaque, with good moisture resistance plus strong chemical and wear resistance and good dielectric properties — the coating to pick where the assembly sees solvents, abrasion or repeated handling.
• Choosing between them: match the coating to the environment the assembly will live in. High voltage diodes that go through thermal cycling benefit from silicone's elasticity, while assemblies exposed to chemicals or mechanical wear usually favour epoxy. Whatever you choose, apply it over the body ends and lead exits, work out air voids, and cure to the compound's own schedule — a coating with voids in it insulates far less than its datasheet suggests.

HV37 Series Voltage Steps

Ordering and Delivery

• Quote by model and quantity: send the model code, the quantity you need and your required date, and we reply with price and availability for that combination.
• Datasheet with the quote: buyers comparing high voltage diode manufacturers usually ask for the ratings table and the package drawing first, so both go out with every reply, together with the installation guide.
• Lead time: typically about 7 days to 6 months depending on quantity — confirm the date for your own order with your sales contact.
• Minimum quantity and payment terms: agreed per order with your sales contact, since they follow the model and the volume you need.
• Samples: ask your sales contact about sample availability if you want a line trial before the production order.
• Packing and shipping: packed for the quantity and destination you specify and shipped by international express. Tell us before dispatch if a consignment needs extra protection for a fragile or high-value assembly.
• Repeat supply: keep the model code and the coating you specified on file, so the next order matches the last one part for part.

Technical Support & Model Matching

Selection starts with your circuit, not a catalogue number: tell us the working voltage, load current, switching frequency and ambient cooling conditions, and our engineers will match a model from our range and explain the margin left in the design. Each model ships with its datasheet, package drawing and installation guide, and we stay available for mounting and soldering questions, start-up checks and fault finding when an existing part runs hot or fails early. Orders run from about 7 days to 6 months depending on quantity, usually by international express.

FAQ